nchrp report 659 – guide for the geometric design of driveways

NATIONALCOOPERATIVE HIGHWAYRESEARCH PROGRAMNCHRP

REPORT 659

Guide for the GeometricDesign of Driveways

TRANSPORTATION RESEARCH BOARD 2010 EXECUTIVE COMMITTEE*

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N A T I O N A L C O O P E R A T I V E H I G H W A Y R E S E A R C H P R O G R A M

NCHRP REPORT 659

Subscriber Categories

Highways • Design • Operations and Traffic Management • Pedestrians and Bicyclists

Guide for the Geometric Designof Driveways

J. L. GattisUNIVERSITY OF ARKANSAS

Fayetteville, AR

Jerome S. GluckAECOM

New York, NY

Janet M. BarlowACCESSIBLE DESIGN FOR THE BLIND

Asheville, NC

Ronald W. EckWEST VIRGINIA UNIVERSITY

Morgantown, WV

William F. HeckerHECKER DESIGN, LLC

Birmingham, AL

Herbert S. LevinsonWallingford, CT

Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration

NATIONAL COOPERATIVE HIGHWAYRESEARCH PROGRAM

Systematic, well-designed research provides the most effective

approach to the solution of many problems facing highway

administrators and engineers. Often, highway problems are of local

interest and can best be studied by highway departments individually

or in cooperation with their state universities and others. However, the

accelerating growth of highway transportation develops increasingly

complex problems of wide interest to highway authorities. These

problems are best studied through a coordinated program of

cooperative research.

In recognition of these needs, the highway administrators of the

American Association of State Highway and Transportation Officials

initiated in 1962 an objective national highway research program

employing modern scientific techniques. This program is supported on

a continuing basis by funds from participating member states of the

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Federal Highway Administration, United States Department of

Transportation.

The Transportation Research Board of the National Academies was

requested by the Association to administer the research program

because of the Board’s recognized objectivity and understanding of

modern research practices. The Board is uniquely suited for this

purpose as it maintains an extensive committee structure from which

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The program is developed on the basis of research needs identified

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and by committees of AASHTO. Each year, specific areas of research

needs to be included in the program are proposed to the National

Research Council and the Board by the American Association of State

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needs are defined by the Board, and qualified research agencies are

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Research Council and the Transportation Research Board.

The needs for highway research are many, and the National

Cooperative Highway Research Program can make significant

contributions to the solution of highway transportation problems of

mutual concern to many responsible groups. The program, however, is

intended to complement rather than to substitute for or duplicate other

highway research programs.

Published reports of the

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM

are available from:

Transportation Research BoardBusiness Office500 Fifth Street, NWWashington, DC 20001

and can be ordered through the Internet at:

http://www.national-academies.org/trb/bookstore

Printed in the United States of America

NCHRP REPORT 659

Project 15-35ISSN 0077-5614ISBN 978-0-309-15473-4Library of Congress Control Number 2010928290

© 2010 National Academy of Sciences. All rights reserved.

COPYRIGHT INFORMATION

Authors herein are responsible for the authenticity of their materials and for obtainingwritten permissions from publishers or persons who own the copyright to any previouslypublished or copyrighted material used herein.

Cooperative Research Programs (CRP) grants permission to reproduce material in thispublication for classroom and not-for-profit purposes. Permission is given with theunderstanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA,FMCSA, FTA, or Transit Development Corporation endorsement of a particular product,method, or practice. It is expected that those reproducing the material in this document foreducational and not-for-profit uses will give appropriate acknowledgment of the source ofany reprinted or reproduced material. For other uses of the material, request permissionfrom CRP.

NOTICE

The project that is the subject of this report was a part of the National Cooperative HighwayResearch Program, conducted by the Transportation Research Board with the approval ofthe Governing Board of the National Research Council.

The members of the technical panel selected to monitor this project and to review thisreport were chosen for their special competencies and with regard for appropriate balance.The report was reviewed by the technical panel and accepted for publication according toprocedures established and overseen by the Transportation Research Board and approvedby the Governing Board of the National Research Council.

The opinions and conclusions expressed or implied in this report are those of theresearchers who performed the research and are not necessarily those of the TransportationResearch Board, the National Research Council, or the program sponsors.

The Transportation Research Board of the National Academies, the National ResearchCouncil, and the sponsors of the National Cooperative Highway Research Program do notendorse products or manufacturers. Trade or manufacturers’ names appear herein solelybecause they are considered essential to the object of the report.

CRP STAFF FOR NCHRP REPORT 659

Christopher W. Jenks, Director, Cooperative Research ProgramsCrawford F. Jencks, Deputy Director, Cooperative Research ProgramsDavid B. Beal, Senior Program Officer, RetiredDavid A. Reynaud, Senior Program OfficerMegan A. Chamberlain, Senior Program AssistantEileen P. Delaney, Director of PublicationsHilary Freer, Senior Editor

NCHRP PROJECT 15-35 PANELField of Design—Area of General Design

Philip B. Demosthenes, Consultant, Denver, CO (Chair)Tom Dodds, South Carolina DOT, Columbia, SC John C. Jones, Georgetown, ME Cynthia Landez, Texas DOT, Austin, TX Rick Laughlin, HDR Engineering, Inc., Sioux Falls, SD Howard R. Ressel, New York State DOT, Rochester, NY Gary Sokolow, Florida DOT, Tallahassee, FL Richard E. Sommer, Urbana, OH Vergil G. Stover, College Station, TX Scott Windley, US Access Board, Washington, DC Joe Bared, FHWA Liaison Richard A. Cunard, TRB Liaison

AUTHOR ACKNOWLEDGMENTS

This guide was developed under NCHRP Project 15-35 by the Department of Civil Engineering at theUniversity of Arkansas (UA), prime contractor, and by AECOM, subcontractor. Subcontractors toAECOM include Accessible Design for the Blind; West Virginia University; Hecker Design, LLC; and Her-bert S. Levinson.

Dr. James L. Gattis, Professor of Civil Engineering at UA, was the Principal Investigator. Jerome S. Gluckof AECOM was the Co-Principal Investigator. Janet M. Barlow (Accessible Design for the Blind), RonaldW. Eck (West Virginia University), William F. Hecker (Hecker Design, LLC), and Herbert S. Levinsonwere special consultants for the project.

C O O P E R A T I V E R E S E A R C H P R O G R A M S

This report presents guidelines that will be of use to state departments of transportation,local governments, and consultants for the geometric design of driveways. It containsdriveway-related terms and definitions, basic geometric controls, a summary of accessspacing principles, and detailed discussions of various geometric design elements. Materialrelated to and supporting the contents of this publication, including an extensive review ofliterature, can be found in NCHRP Web-Only Document 151: Geometric Design of Driveways.(This supporting document is available on the TRB website (www.trb.org), search for“NCHRP Web-Only Document 151”.)

The design of driveways has benefited from little comprehensive research and no nationaldesign guidance since the American Association of State Highway Officials (AASHO) pub-lication, An Informational Guide for Preparing Private Driveway Regulations for Major High-ways, was published in 1959. Since then, roadway design, function, and volumes havechanged as have vehicle design and many other aspects of the roadway environment.

Driveways, especially busy commercial drives, can have a significant impact on the adja-cent roadway. Good driveway design should facilitate smooth vehicle egress and ingress toand from the roadway and should also provide for pedestrians and bicyclists. Drivewaydesign needs to consider the roadway functional class and driveway usage to better accom-modate varying roadway environments, community needs, and existing conditions. Thereis currently little guidance on this issue.

The Draft Guidelines for Accessible Public Rights-of-Way, disseminated by the U.S. AccessBoard for public comment in 2001, provides specific guidelines for such elements as mini-mum width, cross slope, grade, and edge conditions at the intersection of sidewalks anddriveways to comply with the Americans with Disabilities Act. These guidelines are basedon pedestrian needs and do not comprehensively address safe and efficient vehicle move-ments at driveways. Recommendations are needed to accommodate accessibility concernsas well as safe and efficient vehicle use of the driveway.

This research addressed the design of driveways in the form that roadway designers use—the area where the driveway intersects the public road. The objective of this research was todevelop recommendations for the geometric design of driveways that consider standardengineering practice and accessibility needs and provide for safe and efficient travel bymotorists, pedestrians, and bicyclists on the affected roadway. The importance of theseissues is reflected in studies that show that up to 19 percent of reported urban traffic colli-

By David A. ReynaudStaff OfficerTransportation Research Board

F O R E W O R D

sions involve driveway traffic. This design guide was prepared by James Gattis of the Uni-versity of Arkansas and other consultants as a by-product of the research for NCHRP Proj-ect 15-35, “Geometric Design of Driveways.” This research included a literature review, asurvey of street and highway departments, and field studies leading to an improved under-standing of the state of the practice. This guide presents changes to that state of practicebased on the evolving requirements for driveways.

1 Chapter 1 Introduction1 Purpose and Scope of the Guide 1 Need for This Guide 2 Organization and Structure of the Guide 3 References

4 Chapter 2 Terms and Definitions6 References

7 Chapter 3 Design Controls 7 The Driveway Setting 8 User Mix Considerations

10 Attributes of Bicyclists, Drivers, and Pedestrians 10 Motor Vehicle Traffic Attributes 17 References

18 Chapter 4 Driveway Location and Spacing 18 General Guidelines 20 Driveway Location and Spacing 23 References

24 Chapter 5 Geometric Design Elements 26 Sight Distance and Conspicuity29 Bicyclists29 Pedestrians and Pedestrians with Disabilities32 Public Transit Facilities33 Driveway Plan and Cross-Section Elements 55 Driveway Length 65 Driveway Vertical Alignment Elements 75 Other Elements 83 References

C O N T E N T S

Purpose and Scope of the Guide

This document contains guidelines for the geometric design of driveways. The guidelines arean outgrowth of a literature review and synthesis, a survey of state DOTS, and field studies thatwere a part of NCHRP Project 15-35, “Geometric Design of Driveways.” This publication com-plements documents such as the AASHTO Policy on the Geometric Design of Streets and High-ways (1-1) and the Access Management Manual (1-2). This guide is intended for use in both thepublic and private sectors.

The following driveway design objectives guided the authors during the preparation of this guide:

• Provide a safe environment for various users: bicyclists, motorists, and pedestrians (includ-ing pedestrians with disabilities and transit passengers).

• Provide geometry that accommodates the characteristics and limitations of the various users,and avoid geometric conditions that create traffic operations problems.

• Provide driveways that allow traffic to flow smoothly.• Avoid driveway locations that create traffic operations problems.• Provide driveways that are conspicuous and clearly delineated for the various users.

Although it may be impossible to perfectly achieve these objectives, some designs come muchcloser than others in achieving these objectives. Every driveway connection creates an intersection,which creates conflicts with bicyclists, pedestrians, and other motor vehicles. An objective of gooddesign is to seek a balance that minimizes the actual conflicts and accommodates the demands fortravel and access.

Driveways can be defined as private roads that provide access between public ways and activ-ities or buildings on abutting land (1-3). However, when roadway designers use the term “drive-way,” they are often referring to just a part of a driveway—the area where the driveway intersectsthe public highway or street. With few exceptions, the contents of this guide reflect the roadwaydesigner definition of driveway and do not address the design of a driveway well within a privatesite, except as such design affects the driveway intersection with the public roadway. Many ofthese recommendations were prepared to address access connections that are designed to lookmore like the typical driveway rather than those looking like public roadways.

Need for This Guide

Driveways are integral to the roadway-based transportation system. They are found along mostroadways throughout urban, suburban, and rural areas. They range from single-lane connectionsserving single-family residences to multilane, divided-access connections to major activity centers.

1

C H A P T E R 1

Introduction

Driveways vary in size and design according to the activities they serve and the associated traf-fic volumes, development densities, proximity to intersections, and exposure to bicyclists andpedestrians. The design and appearance of driveways have evolved over the years as technolo-gies and land development patterns have changed.

Both anecdotal experience and structured research studies show that certain driveway designpractices create problems for bicyclists, motorists, and pedestrians. Studies have found that any-where from 11 to 19% of all reported urban traffic collisions involve a driveway (1-4). The loca-tion and design of a driveway affect both traffic flow and safety on both the driveway and on theadjacent public roadway.

There has been less study of driveways than of many other types of roadway facilities. Amongthe few publications that have addressed driveway design are the following:

• The American Association of State Highway Officials’ (AASHO) guidelines published in1959 (1-5),

• The Institute of Transportation Engineers (ITE) guidelines published in 1987 (1-6),• Technical assistance from the U.S. Access Board published in 1999 (1-7), and• The TRB Access Management Manual (1-2).

The growing emphasis on multi-modal transportation and the requirements of the Americanswith Disabilities Act (ADA) also call for a re-examination of driveway design practices. Duringpreparation of this document, it became apparent that structured studies and documented infor-mation on which to base recommendations is often limited. It is hoped that future research willhelp improve the knowledge base.

Exhibit 1-1 illustrates some of the operational and safety problems that can arise when drive-way designs are inadequate.

Organization and Structure of the Guide

This guide consists of the following chapters:

• Chapter 2 lists terms and definitions.• Chapter 3 discusses some of the basic geometric design controls. These controls include basic

characteristics of users and vehicles, as well as site-specific controls, such as setting and landuse, types of users, vehicle types, volumes, and speeds. These considerations affect the designpractices recommended in the following chapters.

• Chapter 4 briefly mentions access spacing principles and guidelines and references other pub-lications for more information.

2 Guide for the Geometric Design of Driveways

(a) (b)

Exhibit 1-1. The consequences of driveway design decisions.

• Chapter 5 sets forth various geometric design elements. These include plan and cross sections,driveway length, vertical alignment, and related elements.

Material related to and supporting the contents of this publication, including an extensive reviewof literature, can be found in NCHRP Web-Only Document 151: Geometric Design of Driveways.

References1-1. AASHTO. A Policy on Geometric Design of Highways and Streets. Washington, DC (2004) 896 pp.1-2. TRB Committee on Access Management. Access Management Manual. Transportation Research Board,

National Research Council, Washington, DC (2003) 373 pp.1-3. Webster’s Collegiate Dictionary. G&C Merriam Company, Springfield, MA (1981).1-4. Rawlings, J., and Gattis, J. L. “Detailed Study of Driveway Collision Patterns in an Urban Area.” Compendium

of Papers, 87th Annual Meeting, Transportation Research Board, National Research Council, Washington,DC (2008).

1-5. AASHO. An Informational Guide for Preparing Private Driveway Regulations for Major Highways. Washington,DC (October 10, 1959, copyright 1960) 31 pp.

1-6. ITE. Guidelines for Driveway Location and Design. Washington, DC (1987) 23 pp.1-7. U.S. Access Board. Accessible Public Rights of Way: Design Guide. U.S. Architectural and Transportation

Barriers Compliance Board, Washington, DC (1999).

Introduction 3

This chapter presents terms and definitions used in this report. Exhibit 2-1 illustrates the loca-tion of some of the named driveway design elements.

AASHO – American Association of State Highway OfficialsAASHTO – American Association of State Highway and Transportation OfficialsADA – Americans with Disabilities Act of 1990Blended transition – A connection with a grade of 5% or less between the level of the pedestrian

walkway and the level of the crosswalk (2-1).Breakover angle – The algebraic difference between two successive grades.CBD – Central business district: the established “downtown” core or center of a city that tradi-

tionally included government, office, and retail activities.Commercial driveway – Driveways that serve uses such as offices, retail, or services.Connection – The junction of the subject roadway with a source of traffic from the side (e.g., a

driveway, roadway, or ramp).Contrast – A marked difference between dark and light. With regard to ADA contrast for

detectable warnings, the ADA Standards state the following in the advisory appendix section.A4.29.2 Detectable Warnings on Walking Surfaces. The material used to provide contrastshould contrast by at least 70%. Contrast in percent is determined by:

contrast = [(B1 – B2)/B1] × 100

where B1 = light reflectance value (LRV) of the lighter area and B2 = light reflectance value (LRV) of the darker area.

Note that in any application both white and black are never absolute; thus, B1 neverequals 100 and B2 is always greater than 0.

Cross slope – The slope (or grade) perpendicular to the direction of travel. On a sidewalk orblended transition, it is measured perpendicular to the curb line or roadway edge. On a curbramp, it is measured perpendicular to the longitudinal or running grade.

Driveway triangular island (“pork chop”) – Roadway or driveway channelization in the formof a somewhat-triangular island.

Dust pan –A driveway entry or exit shape with the plan view designed with a flared or tapered edge.With this design, the curb height along the roadway edge transitions from full height to no curbheight. Thus, the design incorporates a taper in both the plan and in the front elevation views.

Front overhang – The distance from the center of the front-most wheel to the front end of thevehicle.

Functional area of intersection – The area that includes not only the physical area where roadwayscross each other, but also the areas upstream and downstream of the physical intersection, wheredriver reaction, deceleration, queuing, and acceleration occur that are related to the operationof the intersection.

Ground clearance – The distance from the bottom of a vehicle body to the ground.

4

C H A P T E R 2

Terms and Definitions

Terms and Definitions 5

Hang-up – When the underside of a vehicle comes into contact with the roadway surface, at gradebreaks in the vertical profile, such that the vehicle is immobilized or stuck on the verticalgeometry. Also referred to as lodged or high-centered.

Interface – The broader area where a driveway joins the roadway, including the curved or flaredturning areas.

ITE – Institute of Transportation EngineersMUTCD – Manual on Uniform Traffic Control Devices. The federal MUTCD is incorporated by

reference in 23 Code of Federal Regulations (CFR), Part 655, Subpart F. It is recognized as thenational standard for all traffic control devices installed on any street, highway, or bicycle trailopen to public travel in accordance with 23 U.S.C. 109(d) and 402(a).

NCHRP – National Cooperative Highway Research ProgramNon-restrictive median – A median designed to be easily crossed by a motor vehicle, such as a

two-way left-turn lane (TWLTL).Offset – The meaning of this term depends on the context. In the context of a driveway connec-

tion transition, it can refer to the situation where due to the presence of on-street parking, abicycle lane, a shoulder, or similar space generally parallel to and outside of the traveled way,the physical end of a driveway is some distance away from the edge of the traveled way. Theeffect of this is that part of the turning movement of those vehicles entering or exiting thedriveway takes place in that area between the edge of the traveled way and the physical end ofthe driveway.

Exhibit 2-1. Some driveway design elements.

PAR – Pedestrian Access RoutePedestrian Access Route – A continuous and unobstructed walkway within a pedestrian circu-

lation path that provides accessibility.Ped – Pedestrian. From the 2003 MUTCD, Section 1A13.55: “a person afoot, in a wheelchair, on

skates, or on a skateboard” (2-2).Pork chop (driveway triangular island) – Roadway or driveway channelization in the form of

a somewhat-triangular island.P-vehicle – The passenger car design vehicle as defined by AASHTO. Also includes minivans,

pick-up trucks, sport utility vehicles (SUVs), and standard size vans.Rear overhang – The distance from the centerline of the rearmost axle to the rear end of the

vehicle.Restrictive median – A median, such as a raised or depressed median, designed not to be crossed

by a motor vehicle except at selected locations.RV – Recreational vehicle (e.g., a motor home).Spillback – When a situation exists such that the traffic conditions at the subject driveway

influence or affect the operation of vehicles in the outside through lane at or in advance of thedriveway upstream of the subject driveway.

TCD – Traffic control devices, including signs, pavement markings, and traffic signalsThreshold – The edge, dividing line, or boundary where the driveway meets the public roadway.

In many cases, this is a line along the curb edge.Throat length – The distance from the outer edge of the traveled way of the intersecting roadway

to the first point along the driveway at which there are conflicting vehicular traffic movements.Also referred to as the driveway connection depth, driveway reservoir length, driveway stack-ing distance, driveway storage length.

Traveled way – The portion of the roadway for movement of vehicles, exclusive of shoulders (2-3).TRB – Transportation Research BoardWheelbase – The distance between the centers of two axles or wheels. Sometimes shown as the

length from the front axle to the rear axle.

References2-1. “R105.5 Defined Terms” in Revised Draft Guidelines for Accessible Public Rights-of-Way. http://www.access-

board.gov/prowac/draft.htm#105 (as of Nov. 23, 2005).2-2. FHWA. Manual on Uniform Traffic Control Devices. Washington, DC (2003) 760 pp.2-3. AASHTO. A Policy on Geometric Design of Highways and Streets. Washington, DC (2004) p. 305.


As with other types of roadway geometric features, the test of how well or how poorly a drive-way connection is designed is determined by how well or how poorly the connection operatesafter it is opened. To anticipate the consequences of a design choice before a facility is actuallyconstructed and opened for use, the designer needs to identify the setting and understand theperformance characteristics and limitations of the users—bicyclists, drivers, pedestrians, andmotor vehicles.

Although there will always be exceptions, the following material describes generally prevalentsituations in the United States. These considerations are incorporated into the more detaileddesign guidelines presented in Chapters 4 and 5.

The Driveway Setting

The design of a driveway is affected by its setting and land use. The environment can be urban,suburban, or rural. The various characteristics of a driveway serving a tract with commercial landuse are quite different from a driveway serving a single-family residence. Combinations of thesecharacteristics and other factors affect the final design choices.

The differences between urban, suburban, and rural settings can be characterized by develop-ment density, the spacing of parallel and intersecting streets, levels of bicycle and pedestrian traf-fic, and the availability of public transit service. In contrast to rural areas, built-up urban areastypically have lower speeds, more frequent intersections, many more pedestrians, and often busservice. In urban settings, especially in central business districts (CBDs), driveway geometry canbe more constrained than in suburban and in rural areas. Exhibit 3-1 lists the relative impor-tance of travel modes, based on the location and development density of the activities to beserved. The relative importance can help the designer determine how to address the sometimesconflicting needs of different modes.

Although all types of property tracts need access to and from public roadways, the nature of thatneed varies according to the type of land use (e.g., agricultural, commercial, and residential). Thetype of land use is typically associated with factors such as the volume of traffic and the types ofvehicles in and out of the driveway. Exhibit 3-2 lists common types of driveways, illustrative appli-cations, and some considerations affecting the design. The organization reflects combinations offactors that designers commonly encounter. “Standard” driveways are grouped by intensity ofuse—very high, higher, medium, and lower. “Special situation” driveways include those that cre-ate special needs (e.g., a driveway in a city center or serving a farm or ranch, a field, or an industry).

Exhibit 3-2 does not list all of the possible combinations of land use and surrounding envi-ronment; a list of all combinations would be extremely complex and unwieldy. The designer

7

C H A P T E R 3

Design Controls

must exercise good judgment that reflects an understanding of traffic characteristics when cat-egorizing a particular driveway and applying design standards. For instance, the small radiusand steep grades that some agencies allow for residential driveways will probably be unsuit-able for a single-family residential driveway connecting to a busy thoroughfare. Land use typealone is not a sufficient criterion for design; the designer must consider other factors, includ-ing the site environment.

User Mix Considerations

Bicyclists, motorized-vehicles (e.g., automobiles, buses), pedestrians, pedestrians with disabil-ities (e.g., persons using mobility aids such as wheelchairs, or pedestrians with visual impair-ments) all occupy and use transportation facilities in the United States. In the area where theroadway, the sidewalk, and the driveway intersect, there are three distinct user groups with dif-ferent and sometimes conflicting needs (see Exhibit 3-3). Although members of each group typ-ically want to make their trips as expeditiously as possible, the roadway user is usually moving ata greater speed and therefore is often focused on the roadway some distance ahead. The sidewalkusers (e.g., pedestrians, pedestrians with mobility disabilities) are moving at a much slower pace,and are unprotected and vulnerable to vehicles. The area may be used by those waiting for a busor taxi. The driveway user typically has a speed and a path that can create conflicts with the othertwo user groups.


Area Type Descriptive Attributes Relative Importance of Mode (as determined by the actual choices the public is observed to make)

Auto Bicycle Ped Transit Urban Core(CBD and other major urban centers)

Connected buildingsSidewalk paved from curb edge to the face of buildingsShorter blocks Higher pedestrian volumes In some locales, higher volumes of transit vehicles and riders Motor vehicle traffic is often congested and moving relatively slowly

Medium Medium High High to Medium

General Urban May include special districts (e.g., outlying business districts that are not in an urban core) Bicycles and pedestrians are present In some locales, public transit vehicles and riders are present

High Medium Medium Medium

Suburban Motor vehicles are predominate mode A few bicycles and pedestrians are presentIn some locales, occasional public transit is present

High Low Low Low

Exurban Motor vehicles are predominate mode Bicycles, pedestrians, and/or transit are infrequent

High Low Low Low

Rural (farm or ranch)

Motor vehicles are predominate mode Higher speeds need longer access spacing

High Very low

Verylow

Verylow

NOTES: “Ped” = Pedestrian“Relative Importance” is affected by region. For instance, public transit is more prevalent in some regions than in others.

Exhibit 3-1. Driveway settings and the importance of various modes.

Design Controls 9

Category Description of Common Applications* Considerations Affecting Design

STANDARD DRIVEWAYS

Very high intensity

Urban activity center, with almost constant driveway use during hours of operation. Typified by a driveway serving a post-1950 major shopping center or office complex. These driveways often look like public roadways. Not uncommon for these driveways to be signalized.

Very high site volume. These sites areoften on streets with relatively high speeds and volumes. For these driveways, refer to street design guides.

Higher intensity

Medium-size office or retail, such as community shopping center, with frequent driveway use during hours of operation. Also includes land uses with extreme peaking patterns, such as public schools, worship assemblies, and employee parking lots.

These sites are often on streets with relatively high speeds and volumes.

Expect more than one exiting vehicle at a time.

Medium intensity

Smaller office or retail, such as convenience stores, with occasional driveway use during hours of operation. Also includes some apartment complexes.

These sites may be on streets with relatively high speeds and volumes.

Seldom more than one exiting vehicle at a time.

Lower intensity

Typical applications include single-family or duplex residential, or other types with low use. May not apply to rural residential.

If on a lower-speed, lower-volume roadway, conflicts with other vehicles are relatively less frequent.

The driveway is used by only one vehicle at a time.

SPECIAL SITUATION DRIVEWAYS

Central business district

Building faces are close to the street. May have on-street parking or bus stops, a continuous sidewalk from the curb to faces of buildings, and higher pedestrian usage than in most other environments. Many situations will serve P-vehicles and some single-unit trucks.

Vehicles entering a driveway may encounter a higher frequency of conflict with other users, such as pedestrians. Through motorists expectmore frequent traffic interruptions.

Farm or ranch

May be a mixture of residential and industrial characteristics, used by a mix of design vehicles, such as P-vehicle, single-unit truck, and agricultural equipment.

May be on a highway with a posted speed of 50 mph or more.

Pedestrian use is extremely rare. The driveway is used by only one

vehicle at a time.

Field (Very low intensity)

Serves a field or other similar land area that is seldom trafficked. Higher-clearance P-vehicles or heavy vehicles are expected.

Many days may elapse between uses. Pedestrian use is extremely rare. The driveway is used by only one

vehicle at a time.

Industrial Driveways frequently used by buses, tractors with semi-trailers, and other vehicles longer and wider than the design P-vehicle.

The extra axles and longer wheelbase will lead to much greater offtracking of vehicles entering the driveway.

Other Identify the specific vehicles that will use the facility. Example – bus terminal

Bus terminal – Consider the width, and swept path of turning buses and circulation patterns.

Example – emergency vehicles Emergency vehicles – Need to exit heading out, not backing. May need on-site turn around.

NOTE: P-vehicle is a passenger car design vehicle, which includes minivans and pickup trucks. * These descriptions are intended to help the designer form a mental image of some of the more common examplesof the category.

Exhibit 3-2. Driveway categories.

These interactions take place within or near the border, the area between theroadway edge and the right-of-way line. Objects in the border can affect theusers. For instance, a poorly placed roadside bus shelter can be an impedimentin the path of a pedestrian with a visual impairment and may make it more dif-ficult for a motorist exiting the driveway to see oncoming traffic.

Driveway design practice should address many issues. Some broad consider-ations include the following:

• Convenient and safe vehicle egress and ingress;• Sight distance and safety for sidewalk users;

Exhibit 3-3. Users in the driveway,roadway, sidewalk area.

Sidewalk users

Drivewayusers

Roadway users

border area

border area

• Accessibility for pedestrians with disabilities and incorporating requirements of the ADAAccessibility Guidelines);

• Interactions where bicycle lanes or paths are present; and• Interactions where public transportation stops are in the vicinity of the driveway.

These considerations affect design details such as sidewalk alignment and cross slope acrossthe driveway, driveway entry shape (curved or straight) and dimension, and driveway width.

Attributes of Bicyclists, Drivers, and Pedestrians

The capabilities and limitations of the people using the driveway, whether as bicyclists, drivers,or pedestrians, affect design choices. An appreciation of the concept of driver work load leads tothe objective of trying to limit the number of (1) decisions a driver has to make and (2) potentialconflicts with different streams of traffic. Acknowledging that rain, fog, and nighttime conditionscan make physical objects more difficult to detect, a designer tries to create well-defined edges andincrease the contrast between different surfaces, such as between the driveway opening and theborder area. Refer to the AASHTO guides for the design of bicycle facilities (3-1), highways andstreets (3-2), and pedestrian facilities (3-3) for a discussion of user characteristics. Characteristicsof Emerging Road and Trail Users and Their Safety (3-4) provides data for a wide range of users,including bicyclists and pedestrians.

Driveways are crossed by pedestrians on sidewalks. Exhibit 3-4 shows a distribution of thewalking speeds of pedestrians under 60 and over 60. In both age groups, most pedestrians walkat speeds between 3 and 6 ft/s.

A synthesis of default values (3-5) cited one study listing 15th percentile walking speeds for thoseless than 60 years old as 3.8 ft/s, and 3.5 ft/s for those over 60. Another study listed 15th percentilewalking speeds for those less than 65 years old as 4.0 ft/s, and 3.1 ft/s for those over 65.

When estimating the time required for a pedestrian to cross the driveway, make an allowancefor the pedestrian not starting from the exact edge of the driveway. A pedestrian may be stand-ing 2 or more feet back from the driveway edge when the pedestrian begins to walk across thedriveway.

Bicyclists also cross the paths of vehicles entering and leaving driveways. On shared use paths(“a bikeway physically separated from motorized vehicular traffic,” the 1999 AASHTO bicycleguide (3-1, p.3) suggested a design speed of at least 20 mph on shared use paths, noting that gradeand wind can affect the speeds of bicyclists. With a downgrade greater than 4%, a design speedof 30 mph or more was offered (3-1, p.36). Discussing urban street design criteria, the UrbanStreet Geometric Design Handbook by ITE (3-6, p.41) stated:

Studies show that nearly all bicyclists travel within a range of 7 to 15 mph, with an average of 10 to 11 mph.

A study that examined characteristics of a wide range of users found that the 85th percentilespeed for bicycles was 14 mph, and for recumbent bicycles was 18 mph (3-4, p.74).

Motor Vehicle Traffic Attributes

The designer should consider the attributes of the motor vehicles used by the drivers. Attributesthat affect driveway design include vehicle width, vehicle length, vehicle height, vehicle turningradius, vehicle off tracking, and vehicle ground clearance dimensions.


Design Vehicles

In its design policy, AASHTO indicates that key controls in roadway geometric design arethe physical characteristics and the percentages of vehicles of various sizes using the roadways.According to AASHTO, it is appropriate to examine all vehicle types, establish general classgroupings, and select vehicles of representative size within each class for design use: Theseselected vehicles, with representative weight, dimensions, and operating characteristics, used toestablish highway design controls for accommodating vehicles of designated classes, are knownas design vehicles (3-2, p.15).

AASHTO identifies general classes of design vehicles and dimensions for design vehicleswithin these general classes. The design policy advises that “the designer should consider thelargest design vehicle likely to use the facility with considerable frequency or a design vehicle withspecial characteristics appropriate to a particular intersection in determining the design of suchcritical features as radii at intersections and radii of turning roadways.” General guidance is givenfor selecting a design vehicle. With one exception (i.e., a passenger car may be selected when themain traffic generator is a parking lot), the guidelines deal with road and street intersections asopposed to driveway-roadway intersections.

Design Vehicle Dimensions

Widths and turning paths of design vehicles can be found in the latest edition of the AASHTOdesign policy. There is some indication that slow-turning vehicles may follow a path with a

Design Controls 11

Exhibit 3-4. Distribution of pedestrian walking speeds.

smaller radius than indicated in the turning dimensions and the turning templates provided inthe current AASHTO Green Book (3-2, pp.16–43).

Underclearance or ground clearance is the distance from the bottom of the vehicle body to theground (3-7). Ground clearance and wheelbase are critical dimensions at a crest situation. Theground clearance, in combination with either the front or rear overhang, is critical at sag situations.For example, rear-load garbage trucks may drag in the rear; therefore, rear overhang is the crit-ical parameter. Car carrier trailers can drag in the rear or hang up between the wheels; therefore,either wheelbase or rear overhang may be critical. When the designer does not take these dimen-sions into account, the result can be vehicles dragging, scraping, and even becoming lodged onthe vertical profile grade changes.

Although a designer can consult the AASHTO design policy for lengths, widths, overall heights,turning radii, and swept path templates for a menu of vehicle types, the policy does not includevehicle ground clearance or underclearance data. Exhibit 3-5 presents vehicle ground clearancedimensions. Note that dashes (—) in cells in the table indicate that hang-up problems are notexpected on this part of the vehicle.

Exhibit 3-6 shows the findings from a recent study in which the underside dimensions of aselect group of vehicles were measured. From this, the crest and sag angles at which undersidedragging would occur were calculated. These values reflect the physical limits of the vehicles.


Exhibit 3-5. Vehicle ground clearance dimensions.

Design Vehicle Rear Overhang

(ft)

Wheelbas e (ft)

Front Overhang

(ft)

Ground Clearance for Rear

Overhang (in)

Ground Clearance

for Wheelbas e

(in)

Ground Clearance for Front Overhang

(in)

Rear-Load Garbage Truck 10.5 20 -- 14 12 --

Aerial Fire Truck 12 20 7 10 9 11

Pumper Fire Truck 10 22 8 10 7 8

Single-Unit Beverage Truck 10 24 -- 8 6 --

Articulated Beverage Truck -- 30 -- -- 10 --

Low-Boy Trailer <53 feet -- 38 -- -- 5 --

Double-Drop Trailer -- 40 -- -- 6 --

Car Carrier Trailer 14 40 -- 6 4 --

Belly Dump Trailer -- 40 -- -- 11 --

Mini-Bus 16 15 -- 8 10 --

School Bus 13 23 -- 11 7 --

Single-Unit Transit Bus -- 25 18 -- 8 6

Motorcoach 10 27 7.6 8 7 10

Articulated Transit Bus 10 -- -- 9 -- --

Passenger Vehicle and Trailer - Private Use

13 20* -- 5 5 --

Passenger Vehicle and Trailer - Commercial Use

13 24* -- 7 7 --

Recreational Vehicle (RV) 16 27 7.8 8 7 6

NOTES: * indicates distance from rear wheels to hitch -- indicates hang-up problems not expected on this part of the vehicle These dimensions reflect only the physical limits of vehicles. They do not account for the effects on vehicles in operation (e.g., dynamic load—vehicle bounce). The desirable maximum grade changes will be less than those reflected in these values.

Design Controls 13

Exhibit 3-6. Ground clearance geometry for specific models.

GSAG = 15.0%

GCREST = 13.5%

These calculations do not account for effects of static load (weightof passengers or cargo) or dynamic load (vehicle bounce).Maximum desirable grade change will be less than these values.

GSAG = 15.0%

GCREST = 13.5%

P-CAR: based onChevrolet Camaro 1998Chevrolet Corvette Z06 2008

GSAG = 13.9%

GCREST = 18.9%

TRACTOR WITH 10-BAY BEVERAGETRAILER: based onInternational tractor, Centennial Body trailer,about 5/8 loaded

CLASS A DIESEL MOTOR HOME (DIESEL PUSHER): based onAlfa See Ya’! Gold®

GSAG = 13.9%

GCREST = 18.9%

GSAG = 15.0%

GCREST = 13.5%

GSAG = 15.0%

GCREST = 13.5%

PICKUP TRUCK WITH TRAILER: based onFord F-150 with Wells Cargo 32 ft two-axleball-hitch trailer

GSAG = 7.0%

GCREST = 13.0%

Pick-up with trailer and beverage truck calculations by R. Eck.Passenger car and motor home calculations by J. Gattis.

Angles used for design, reflecting attributes of vehicles under actual operation conditions, shouldbe less than these.

Selecting a Design Vehicle

The activities served and the location of a driveway will affect the types of vehicles using thedriveway. Typical vehicles include passenger cars, service vehicles, and bicycles. Large trucks,with their wide offtracking, use many commercial driveways—although usually few in number,larger trucks must be able to negotiate curves and grades. They should be the design vehicle fordriveways serving industrial areas.

Design vehicle selection involves two conflicting mandates: (1) select a vehicle with sufficientlylarge dimensions so that all users can negotiate the driveway in the future and (2) confine thedimensions so that the driveway is not overdesigned. Designers can easily believe that they lackinformation needed to select a design vehicle. Designers may not know how frequently certainlarger vehicles will use a site; regardless, the word “considerable” in the phrase “use . . . with con-siderable frequency” is undefined. Designers are left to their judgment to assess to what extent it isacceptable for offtracking turning vehicles to encroach into other lanes. Not only is the frequencyof vehicle use a consideration, but the volume and speeds on the main roadway are also factors.

Exhibit 3-7 lists suggested design vehicles for various types of driveways. Exhibit 3-8 shows anexample from a state transportation agency.

Vehicles for Farm/Ranch and Field Entrance Design

Design vehicle information for farm vehicles is not generally available. The County Engineerfor Delaware County, Iowa, Mark J. Nahra, P.E., observed that large equipment will be foundusing both the field entrances and driveways to farm residences. Also, P-vehicles use fieldentrances, so the designer should use both the standard driver eye height for a P-vehicle and theeye height for a heavy vehicle.

Despite their size, large combines and other pieces of farm equipment are very maneuver-able. Large combines are usually less than 16 feet wide. Based on this, farm driveways and

field entrances should be at least 16 feet wide, although 20 feet is recommended. A 30-foottop-width over the driveway culvert is recommended to allow large combines and tractor-semitrailer combinations to pull into farm driveways. A radius of at least 20 feet is recom-mended to allow service vehicles (e.g., propane or fuel oil trucks) (single-unit vehicles) to beable to turn safely into a rural residential driveway. A site review is recommended to assessground clearance issues.


Exhibit 3-7. Suggested design vehicles for common driveway types.

Category Description of Common Applications Design Vehicles

STANDARD DRIVEWAYS

Very high intensity

Urban activity center, with almost constant driveway use during hours of operation.

Large truck, buses (May be P-vehicle if have separate truck entrances.)

Higherintensity

Medium-size office or retail (e.g., a community shopping center) with frequent driveway use during hours of operation.

Large truck, buses (May be P-vehicle if have separate truck entrances.)

Medium intensity

Smaller office or retail, some apartment complexes, with occasional driveway use during hours of operation.

P-vehicle, single-unit truck

Lowerintensity

Single-family or duplex residential, other types with low use. May not apply to rural residential.

P-vehicle


Parking lot or garage for automobiles only P-vehicle Centralbusinessdistrict Other than exclusive automobile facility Single-unit truck

Farm or ranch

Single-unit truck, farm equipment

Field Seldom used, very low volume Single-unit truck, farm equipment

Industrial Driveways are often used by large vehicles Large truck

Other Bus terminal Bus Fire or Ambulance station Emergency vehicle

Notes: P-vehicle is the AASHTO passenger car design vehicle. Large truck may be WB-50, WB-62, or WB-65. These descriptions are intended to help the designer from a mental image of some of the more common examples of the category.

Mix of residential and industrialcharacteristics

Exhibit 3-8. Example design vehicles for driveway types.

Design Volumes

Estimating the expected driveway volume can help identify how many driveway lanes areneeded. For more information, refer to publications that discuss methodology for site impactstudies. The basic steps are as follows:

1. Establish the extent to which access is allowed, and estimate the number of driveways. Reviewthe local access policy and spacing standards of the governing agency to establish whether thedesired access will be allowed and, if so, to identify the number of driveways.

2. Identify the type and size of land use activity to be served.3. Determine the daily and peak-hour vehicle trip rates. If the site currently exists and traffic vol-

umes are expected to remain the same, then counts of existing traffic can be made. For proposeddevelopment, the designer may use ITE Trip Generation or locally developed trip generationrates. By definition, median rates are exceeded 50% of the time, so it may be desirable to calcu-late and use the 80th to 90th percentile trip rates rather than using median or average rates. Ifdriveways are or will be in the CBD or outlying urban business districts, some person trips toor from activities may be made as pedestrians or via public transit. In these cases, some down-ward adjustment of published ITE vehicle trip rates may be warranted.

4. Estimate the daily and peak-hour trips ends for the activity. Multiply the trip generation rateby the appropriate independent variable to arrive at the total number of expected trip ends.

5. Estimate the driveway volumes. Based on the preceding steps, estimate how much site trafficwill use each driveway.

Exhibit 3-9 lists examples of land uses and their expected number of driveway trips.

Design Speeds

Various factors, including the setting and the functional classification, will affect the designspeed of a given roadway. After a roadway has been constructed and is in operation, actual speedson the roadway can be observed. The speeds on the through roadway will normally govern geo-metric features such as sight distance and the length of acceleration or deceleration lanes.

A few studies have measured the speeds at which drivers turn into driveways or side streets.Studies of turning behavior have reported speeds of less than 15 mph for a radius of 30 ft or less.Different studies may measure speeds at different locations or over different lengths during aturn. Exhibit 3-10 shows findings from an older study.

In 2007 and 2008, the speeds of over 1500 vehicles entering 12 driveways were measured nearthe roadway-driveway intersection and in the driveway throat (see Exhibit 3-11). All of the siteswere lower-intensity commercial (e.g., retail and professional offices) developments in built-upsuburban environments along multilane arterial roadways with either 40-mph or 45-mph postedspeed limits. The right-turn entry radii ranged from 13 to 19.5 ft. Almost all of the vehicles inthe study were passenger cars. Sidewalks were present at all sites, but pedestrian volumes were

Design Controls 15

Exhibit 3-9. Estimated number of trips from given sites.

Example Land Uses Expected Number of Site Trips 150,000 sq. ft. shopping center Grocery/drugstore with 10-15 smaller stores (9,000 daily trips split w/2 driveways )

Over 4,000 trips/day or over 400 trips/hour

Small “strip” shopping center (20,000-75,000 sq. ft.) Gas station/convenience market

601-4,000 trips/day or 61-400 trips/hour

3 to 60 housing or apartment units Small office in converted home “Mom and pop” business

21-600 trips/day or 6-60 trips/hour

1 or 2 single-family homes 1-20 trips/day or 1-5 trips/hour

Source: Driveway Handbook, Florida Dept. of Transportation, March 2005


Exhibit 3-10. Driveway entry speed relatedto driveway radius and width.

Exhibit 3-11. Measured speeds of vehicles entering driveways.

Land Use Entry Entry Location Rt. Turn Lt. Turn Type Lane Radius Where Entry 90th% Entry 90th% Width Measured Speed Speed (ft) (ft) (mph) (mph)

Commercial 13 ft 13.0-19.5 2Rt 15.5 to 18.0 naCommercial 13 ft 13.0-19.5 2Lt na 10.0 to 13.0 Commercial 13 ft 13.0-19.5 4 7.0 to 10.4 7.8 to 13.9

LOCATION NOTES -- Speeds measured at:2Rt-right turn, 25 ft before the near perpendicular edge of the driveway 2Lt-left turn, one lane width in advance of the driveway threshold (curb line)4-in the driveway throat, 15 ft. past the driveway threshold (curb line) NOTE: “na” = not applicable

25’

4

11 ’

15’

2 Lt

2 Rt

25’

ROADWAY

4

11 ’

15’

2 Lt

2 Rt

very low. None of the measurements were taken at regional shopping centers or other similarlarge urban activity centers.

At a point 25 feet in advance of the near edge of the driveway, 90% of drivers about to turnright had decelerated to between 15.5 and 18.0 mph or less. Only 10% of drivers turning left hadmeasured speeds of more than 10.0 to 13.0 mph when their vehicles were one lane away fromthe driveway end. At approximately the position at which the rear bumper had cleared the road-way, 90th percentile speeds ranged from 7.0 to 13.9 mph.

References3-1. AASHTO. Guide for the Development of Bicycle Facilities. Washington, DC (1999) 78 pp.3-2. AASHTO. A Policy on Geometric Design of Highways and Streets. Washington, DC (2004) 896 pp.3-3. AASHTO. Guide for the Planning, Design, and Operation of Pedestrian Facilities. Washington, DC (July 2004)

127 pp.3-4. Landis, B. W., Petritsch, T. A., and Huang, H. F. Characteristics of Emerging Road and Trail Users and Their

Safety. FHWA-HRT-04-103 (October 2004) 117 pp.3-5. Zegeer, J. D., et al. NCHRP Report 599: Default Values for Highway Capacity and Level of Service Analyses.

Transportation Research Board, National Research Council, Washington, DC (2008) pp. 55–58.3-6. Elizer, R. M. “Chapter. 2: Design Philosophy and Controls.” Urban Street Geometric Design Handbook, ITE,

Washington, DC (2008) p. 41.3-7. French, L. J., Clawson, A., and Eck, R. W. “Development of Design Vehicles for the Hang-Up Problem.”

Transportation Research Record 1847, Transportation Research Board, National Research Council,Washington, DC (2003) pp. 11–19.

Design Controls 17

18

For many decades, knowledgeable transportation professionals have recognized the need tomanage access along roadways to preserve safety and mobility (see Exhibit 4-1 for an example).In practice, this includes regulating the number of, location of, spacing between, and geometricdesign of driveways.

Several access management guidelines have been developed to assist agencies in balancing thecompeting needs for mobility along the roadways and access to abutting land developments. Oneof the most complete sources of information is the Access Management Manual (4-1). Othersalient guidelines are contained in

• NCHRP Report 348: Access Management Guidelines for Activity Centers (4-2),• NCHRP Report 420: Impacts of Access Management Techniques (4-3),• Transportation and Land Development (4-4), and• A Policy on Geometric Design of Highways and Streets, the AASHTO Green Book (4-5).

Given that access management is addressed by other publications, this design guide willonly briefly discuss the topic. For more information, refer to access management publicationsand websites.

General Guidelines

Although private property enjoys the right of access to the general system of public roadways,this is not an unlimited right. The right of access must be balanced with the needs of and poten-tial harm to the general traveling public. To preserve mobility and provide safety for the travel-ing public, many transportation agencies have established regulations and programs to manageaccess to their roadway network. The regulations are more restrictive for major arterials, theroadways intended to accommodate higher volumes and speeds; however, some objectives andpractices apply to most driveways.

Access management programs restrict the number of driveways allowed. These practices affectwhen and where direct driveway access will be allowed onto the roadway network, whether alter-native access should be provided, and the need for shared access. If direct access is allowed, theguidance includes the extent of that access (i.e., right-in and right-out versus full movement) andcircumstances in which multiple driveways are allowed. In addition, agencies may require thatsteps be taken to mitigate projected traffic operations and/or safety impacts. An example of mit-igation would be providing an auxiliary lane to remove driveway turning traffic from the throughtraffic lanes on an arterial.

As noted in the AASHTO Green Book (4-5, p.729), driveways should not be located withinthe functional area of an intersection or in the influence area of an adjacent driveway. The func-

C H A P T E R 4

Driveway Location and Spacing

Driveway Location and Spacing 19

tional area extends both upstream and downstream from the physical intersection area andincludes the longitudinal limits of auxiliary lanes. As a result, the functional area encom-passes the area where motorists are responding to the intersection, decelerating, andmaneuvering into the appropriate lane to stop or complete a turn. The AASHTO GreenBook also notes that a driveway influence area includes the following:

• Impact length (the distance back from a driveway that cars begin to be affected by drive-way traffic),

• Perception-reaction distance, and• Vehicle length.

Additional guidance related to computing driveway influence areas is available inNCHRP Report 420: Impacts of Access Management Techniques (4-3, pp.48–62).

Another general guideline that applies to driveway location is that sight distance mustbe sufficient. The AASHTO Green Book (4-5, pp.110–155 and 651–677) contains detailedguidance on the purpose and computation of sight distance. In addition, driveways mustbe located so that they are conspicuous and clearly delineated for the various users.

One major objective is to avoid driveway queuing that backs up into a public roadway. Thisis accomplished through design of the throat length, internal circulation, and traffic controlwithin a site. Queuing of traffic exiting a site does not affect the operation of the public roadway,but could affect site circulation and parking lot operations. This internal queuing is affected bythe throat length, number of egress lanes, and traffic control at the public roadway intersection.

Exhibit 4-2 illustrates the confusion and potential for crashes when vehicles slow, changelanes, and try to enter or exit driveways that are too close to each other. Exhibit 4-3 clearly showsthe increased potential for traffic conflicts when driveways are too close to the intersection of twopublic roadways. Exhibit 4-4 shows a vehicle conflict resulting from a driveway too close to theexit ramp off of a freeway.

General guidelines often applied by agencies deciding whether to allow or deny access follow:

• Along the main roadways, limit the number of access points. Encourage property access fromsecondary roads and streets or “backage” roads.

• One carefully located and well-designed driveway per site is often adequate.

Lack of access control along arterial highways is the largest single factor resulting in functional obsolescence of highway facilities. Frequent driveways and curb cuts increase points of conflict and potential accident locations…. Few cities in the United States and Canada exercise effective access control along arterial streets … restrictions on driveway location and spacing are frequently minimal and the criteria are loose.

Marks, H. Traffic Circulation Planning forCommunities, Gruen Associates,Los Angeles, CA (1974) p. 232.

Exhibit 4-1. Experts have long recognized the deficientstate of the practice.

(a) (b)

Exhibit 4-2. Driveways too close to each other allow more conflicts to occur.

Exhibit 4-4. Driveways too close to exit ramp terminals allow more conflicts to occur.


Exhibit 4-3. Driveways too close to roadway intersections allow more conflicts to occur.

• Where two lower-volume sites are adjacent, access to both can be provided by a single shareddriveway. When access from the major roadway is required, sharing access with adjacent tractsreduces the overall number of connections to the major roadway. Shared access arrangementshould be implemented by an appropriate joint easement.

• For higher volume sites, additional access points may be needed. The assessment of this needmust consider (1) whether or not good site planning principles have been applied and (2) thetraffic safety and operational effects of the additional access.

• Along major roadways, the left-turn exit movement from driveways should be kept to a min-imum. If a roadway is converted from undivided to divided, left-turn access may be closed inone or both directions. Where physically practical, direct left-turns can be replaced by rightturns followed by u-turn movements.

When access is not available from parallel or cross streets, or across adjacent tracts, it may benecessary to provide property access from the major roadway. This access often should be lim-ited to right turns only. However, in some situations, limiting access to only right turns will resultin left-turning movements migrating to and overloading a nearby intersection—in such cases,it may be better to allow left-turn movements at the subject access point. An assessment may beneeded as to which arrangement helps preserve the functionality of the roadway and the mobil-ity of the traffic.

Driveway Location and Spacing

Experience has shown that certain driveway locations tend to be problematic, and that for bet-ter safety and mobility, the frequency of driveways should be minimized. This section discussesthe following four types of driveway spacing:

• Spacing between unsignalized connections;• Spacing of driveways from signalized intersections (corner clearance);• Spacing for a signalized driveway; and• Spacing of a driveway from an interchange ramp.

Spacing Between Unsignalized Connections

Spacing between unsignalized connections (whether between two driveways or a drivewayand a roadway) should not interfere with safe and relatively unimpeded movement on thethrough roadway. Driveway spacing practices should provide reasonable access to abuttingprivate property. General guidelines pertaining to unsignalized driveway spacing follow:

• The needed distance between successive connections (both driveways and side streets) increaseswith higher operating speeds, higher access classifications for the public roadway, and higherdriveway volumes.

• A driveway should not be located within the functional area of an intersection or in the influ-ence area of the upstream and downstream driveways.

• Left-turn lane storage requirements should be considered when determining the drivewayinfluence area and can limit how closely driveways can be spaced.

• On roadways that are undivided or have TWLTLs, the alignment of driveways on oppositesides of the road needs to be considered. Driveways on opposite sides of a lower-volume road-way may be aligned across from each other. Alternatively, they should be spaced so that thosedrivers desiring to travel between the driveways on opposing sides of the roadway need tomake a distinct right turn followed by a left turn (or a left followed by a right). A much longerseparation is needed on a higher-speed, higher-volume roadway (4-4).

• On roadways with restrictive medians, the spacing between right-turn access points on oppo-site sides of the road can be treated separately.

• Ideally, driveway access for a major development involving left-turn egress movements shouldbe located where effective coordination of traffic signals would be achievable if there is a needto signalize the driveway.

• Driveway connections to public roadways are subject to the same intersection control deviceanalyses as are street intersections. If existing or future volumes warrant installing a traffic sig-nal, and signalized spacing requirements cannot be met, left-turn access should be subject toclosure in one or both directions.

Driveway spacing from roundabout considerations are similar to those of other types of inter-sections, but driveways may be closer to a roundabout because of shorter queuing. Drivewaysshould not interfere with operation of the roundabout.

General guidelines for unsignalized access spacing are contained in the Access ManagementManual (4-1) and NCHRP Report 348 (4-2).

Spacing of Driveways from Signalized Intersections

The needed minimum separation distance (i.e., corner clearance) from a driveway to a signal-ized upstream or downstream location will depend on the function, operation, and design featuresof the roadway and the characteristics of the access connection. The basic principle of locating oneconnection outside of the functional area of another connection applies to driveways.

For a driveway upstream of or approaching a signalized location on a major road, the func-tional area includes the perception-reaction time, maneuver distance, and storage length of thetraffic on that approach. The spacing should provide separation between the conflicting move-ments occurring at the signal and the conflicting movements occurring at the driveway. In addi-tion, this spacing would enable the driveway to operate without being obstructed by the trafficbacking up from the signal.


Speed (mph)

Cycle length

25 30 35 40 45 50 55

60 1100 1320 1540 1760 1980 2200 2420 70 1280 1540 1800 2050 2310 2570 2820 80 1470 1760 2050 2350 2640 2640 2640 90 1630 1980 2310 2640 2640 2640 2640 120 2200 2640 2640 2640 2640 2640 2640

NOTES: Spacing distances are in feet. Where the recommended spacing in the table exceeds ½ mile (2,640 ft), designers can limit the actual spacing to 2,640 ft.

Exhibit 4-5. Signalized intersection spacing forvarious progression speeds and cycle lengths.


The spacing for a driveway downstream of the departure leg (i.e., far side) of a signalized locationon a major road should be sufficient to minimize the adverse effects of the driveway operations onthe intersection. According to Transportation and Land Development (4-4, p.6–28), the minimumdownstream corner clearance should be no less than the stopping sight distance.

Along the far side of an intersection of a crossroad with an arterial, the corner clearancedistance to the first driveway varies. If the arterial does not have a channelized right-turn lanefor traffic turning onto the crossroad, one source recommends that the driveway be spaced aminimum of 120 feet from the intersection. If the arterial has a channelized right-turn lanefor traffic turning onto the crossroad, the clearance distance should reflect the inside cornerradius. The clearance should be 200 feet for a 50-foot radius, 230 feet for a 75-foot radius,and 275 feet for a 100-foot radius (4-4, p. 6–35). The stopping sight distance principle alsoapplies to driveways connecting to crossroads, along the far side of the intersections of cross-roads with major roads.

For crossroads, the near side corner clearance should extend beyond the normal queuing dis-tance along the crossroad.

Spacing for a Signalized Driveway

Signal spacing is a function of travel speed and signal cycle length. The same criteria for sig-nal spacing apply to both a signalized driveway and a signalized public roadway intersection.If a driveway is going to be signalized, then it should be located to “fit” into the traffic signalprogression along an arterial roadway and not interfere with the progression of traffic fromone signalized intersection to the next.

Desirable spacing is shown in Exhibit 4-5. When signalized driveways and intersections canbe placed at these distances, there is no loss in green-band (through band) width. Small devia-tions (e.g., less than 10%) will have minimal negative effects on the progression. Further guidelinesfor green-band width are contained in NCHRP Report 348 (4-2, p.56–58) and in the AccessManagement Manual (4-1, p.140–149).

Where the recommended spacing in the table exceeds 1⁄2 mile (2,640 feet), designers can limitthe actual spacing to 2,640 feet.

Spacing of a Driveway from an Interchange Ramp

The needed driveway separation distance from an interchange area depends on the geomet-ric design of and methods of traffic control at the freeway ramp joining the roadway. It is alsoaffected by the speed, volume, and number of lanes on the through roadway, the ramp volume

and speed, the number of vehicles turning into the driveway, the type of traffic control at thedriveway, and whether the subject driveway is on the same or opposite side of the road fromthe entry ramp.

Where the ramp entry is signalized, signal spacing criteria should govern where access con-nections are provided. A time-space analysis of the signals along the arterial, including any rampsignalization, can help in identifying the best locations for signalized access.

Unsignalized ramp entry junctions can be either stop- or yield-controlled, with a geometry thatis either free flowing or one that forces the ramp vehicle to come to a stop before entering the road-way. If a driveway is too close to an upstream ramp that is entering an arterial, this can cause con-gestion with spillback onto the ramp and additional conflict on the through roadway segment.This concern can be heightened where there is insufficient distance for the following sequenceto occur: vehicles exit the ramp, merge into the outside lane of a multilane arterial, weave acrossthrough travel lanes, and finally enter an inside or left-turn lane to turn into a driveway on theopposite side of the roadway from the ramp. Vehicles making this maneuver have to wait for gapsin the through traffic lanes before weaving to the left. At locations with higher volumes, higherspeeds, or free-flow movements from the ramp to the roadway, a longer distance is required tosafely make this maneuver. NCHRP Report 420 is one source of spacing guidelines (4-3).

References4-1. TRB Committee on Access Management. Access Management Manual. Transportation Research Board,

National Research Council, Washington, DC (2003) 388 pp.4-2. Koepke, F. J., and Levinson, H. R. NCHRP Report 348: Access Management Guidelines for Activity Centers.

Transportation Research Board, National Research Council, Washington, DC (1992) 111 pp.4-3. Gluck, J. S., Levinson, H. R., and Stover, V. G. NCHRP Report 420: Impacts of Access Management

Techniques. Transportation Research Board, National Research Council, Washington, DC (1999) 157 pp.4-4. Stover, V. G., and Koepke, F. J. Transportation and Land Development, 2nd edition. ITE (2002) 700 pp.4-5. AASHTO. A Policy on Geometric Design of Highways and Streets. Washington, DC (2004) 896 pp.


24

This chapter sets forth geometric design concepts and guidelines for various driveway designfeatures and components. Exhibit 5-1 suggests that driveways created as afterthoughts are lesslikely to perform well. The design of a driveway should be integrated into and take place duringthe design of the overall site. Before the overall site design is finalized, it may need to be adjustedand readjusted so as to have an acceptable driveway design.

Exhibits 5-2 and 5-3 list geometric design elements that a designer may need to consider; not allelements will be present in every situation. This chapter groups some of these driveway geometricelements into the sections listed below and presents specific guidelines and suggested dimensions:

• Driveway throat transition geometry• Driveway width and number of lanes• Median in driveway• Right turn channelization in the driveway• Channelization in the street• Cross slope• Horizontal alignment• Intersection angle• Space for nonmotorized users• Driveway edge and border treatments• Clearance from fixed objects• Length• Driveway grade (sidewalk cross slope), change of grade, and vertical alignment• Sidewalk cross slope (driveway grade)• Roadway-driveway threshold treatment• Drainage of surfaces occupied by user groups• Auxiliary right-turn lanes

Presenting separate design guidelines for every conceivable combination of factors would makea publication unwieldy and overwhelm the user. For instance, when discussing the minimum con-nection transition radius needed for a residential driveway, not only is the width of the drivewayimportant, but the needed radius is also affected by the width of the roadway and the absence orpresence of on-street parking on one or both sides of that roadway. Therefore, the authors havepresented recommendations suitable for more commonly encountered scenarios. Some of theguidelines may not apply to unusual situations.

C H A P T E R 5

Geometric Design Elements

Geometric Design Elements 25

Exhibit 5-1. Unacceptable driveway designs.

Shared Elements, Surroundings1 Illumination2 Conspicuity (to visually detect an element at a distance) 3 Sight obstructions

Driveway4 Width (maximum and minimum; sufficient for ped. refuge)5 Lanes (number, width)6 Median in driveway: (absence or presence)7 width8 type (raised, flush, depressed)9 nose-end recessed from edge of through-road

10 Cross slope, cross slope transition runoff11 Horizontal alignment, curvature12 Connection depth (throat length)13 Traffic controls or other potential impediments to inbound traffic (inc'l entry gate) 14 Paving length (applicable where have unpaved driveway)15 Onsite turnaround capability (where backing into roadway is undesirable) 16 Driveway edge (edge drop off, barrier)17 Space for nonmotorized users (e.g., pedestrian movement parallel to driveway) 18 Driveway border treatments (sideclearance, sideslope)

Vertical profile19 grade (maximum and minimum)20 change of grade (grade breaks)21 vertical curve design criteria22 Vertical clearance (from overhead structures, utility lines)23 Drainage (separate from intersection drainage)24 Other special situations (e.g., railroad crossing, trail, bridle path, etc.)

Sidewalk-Driveway Intersection25 Sidewalk cross slope (i.e., driveway grade)26 Path definition (e.g., visual, tactile cues)27 Crossing length (i.e., driveway width)28 Angle of intersection with driveway:

flat-angle (turn angle < 90O); right-angle (turn angle 90O); sharp-angle (turn angle > 90O)29 Bearing of sidewalk relative to street (i.e., sidewalk diverging from, parallel to, or converging with the street)30 Grade of sidewalk (i.e., driveway cross slope)31 Vertical profile of pedestrian route (abrupt elevation change: max. 1/4" )32 Sidewalk-driveway interface treatment: i.e., detectable warnings for visually impaired (e.g., truncated dome)

(only at certain locations, inc'l. at signalized crossing; refer to guidelines)

Exhibit 5-2. Driveway geometric design considerations that may be within the control of the designer.

(continued on next page)


Sight Distance and Conspicuity

Two considerations that are frequently part of the discussion of many design elements aresight distance and conspicuity.

There are many types of sight distance. The basics of stopping sight distance and intersectionsight distance are explained in the AASHTO Green Book (5-1, pp. 109–114 and 650–676), andan understanding of these basics is a mandatory prerequisite for anyone designing a roadway ordriveway connection to a roadway. The designer should check that walls, wide utility poles, veg-etation, or other objects do not block the lines of sight that a bicyclist, driver, or pedestrian needsto maneuver safely.

Conspicuity is the attribute of standing out so as to be noticed or observed. As applied to drive-ways, conspicuity means that users (whether bicyclists, motorists, or pedestrians) approachingthe driveway can detect and recognize the presence of the driveway far enough in advance soas to make any needed adjustments in their travel trajectory or speed. Also, as the user either onthe roadway or on the “private side” nears the driveway, the user can detect the precise edge or

Exhibit 5-2. (Continued).

Tr affic Controls (for driv ew ay ve hicles) 48 Driv ew ay -road wa y intersection co ntro l ( none, yi eld, st op, signal) 49 Tu rn restri ct ions 50 O ne-wa y operation (one- wa y, do not enter ) 51 Mark ings (pavem ent, delineator s) 52 Ot he r

Road wa y in Vicinity of the Dri ve wa y 53 Right-turn lane attributes: (absence or presence ) 54 lane width 55 lane deceleration, storage length 56 lane entry transition shape 57 lane offset 58 Left-turn lane attributes : ( abs enc e or presence ) 59 lane width 60 lane deceleration, storage length 61 lane entry transition shape 62 lane offset 63 Num ber of driveway s per site 64 Driv ew ay sp acing from upstream access connectio n 65 Driv ew ay sp acing from down st r eam access connection

Ro ad wa y- Drivew ay Inte rs ection 33 Angle of intersection wi th street :

fl at-a n g le ( turn an g le < 90 O ) ; ri g ht-a n g le ( turn an g le 90 O ) ; shar p -a n g le ( turn an g le > 90 O ) 34 Cross slope of st reet and shoulder, considered wi th drivew ay grade 35 Curb threshold treat me nt (rolled, vertical lip, counterslope, continuous ) 36 Curb-ter mi nation treat me nt (abrupt end, drop-dow n, returned) 37 Entry transition shap e ( e.g., radius, flare/taper, straight)38 Entry transition-shape dim ensions (radius, flare dim ensions) 39 Channeliz ation of right turn fr om street into drivew ay 40 Channeliz ation of right turn fr om drivew ay into street 41 Channeliz ation in the driveway (e.g., triangular island to prohibit in and out left-turns) 42 Channeliz ation in st reet - street me dian prohibits all le ft -turns in/out of driveway 43 Channeliz ation in st reet - street me dian prohibits one but not both le ft -turns 44 Drainage: confining the gutter flow 45 Drainage: inlet ty pe and location 46 Clearance from fix ed objects, appurtenance s 47 Pavem ent su rface defor mi ty (corrugation, potholes )

other elements that will affect the user’s position and path when crossing, entering, or exiting thedriveway. Means to improve conspicuity include the following:

• Clearly defining the edges of shapes, so as to differentiate between shapes (e.g., the edge betweena sidewalk and a driveway);

• Providing contrast between light and dark surfaces;• Placing a business sign near the driveway, to reinforce its location; and• Installing artificial illumination.

Exhibits 5-4 and 5-5 show undesirable design practices. Exhibit 5-4 shows how a planter anda utility pole near the intersection of a driveway with a roadway restrict the sight distance. Visual

Exhibit 5-3. Driveway considerations generally outside the control of the designer.

Shared Elements, Surroundings 1 Land us e 2 User and vehicle mi x and co mp os ition 3 Te mp oral variation: seas on, da y of week , tim e of da y 4 W eather and weather ef fe ct s

Side wa lk -D riv ew ay Intersectio n 5 Sidewalk plac em en t ( adj ac ent to or o ffs et fr om the cu rb or edge)

Roadw ay -Dri ve wa y Intersec tion 6 Elevation dif fe rence betw een roadway surface and abutting property

Roadway in Vicinity of the Driveway 7 Wi dth of roadway 8 Lanes (num ber, width) 9 Lane ty pe (tra ve l, HOV, bic yc le, turn, par ki ng)

10 Cross slop e ( travel lanes, shoulders ) 11 Hori zo ntal alignm ent of roadway 12 Vertic al prof ile of roadway 13 Sight distanc e re st riction s

User Characteristics - Bicyclist 14 Bic yc lis t perception-r eac tion proc ess, tim e 15 Spee d 16 Brak ing capabilit y 17 Sight distanc e nee d

User Characteristics - Pedestrian 18 Pedestrian perc eption-reac tion process, ti me 19 Spee d 20 Sight distanc e nee d

Spec ial needs groups 21 General - children, elderly 22 Those with disabilities (e.g., mobility, visual)23 Legal mandates - those related to disabilities

User Characteristics - Vehicle, Dri ve r 24 Driv er perception-reaction process, ti me 25 Spee d 26 Deceleration characteristics (ty pical) 27 Brak ing capabilit y (lim iting) 28 Sight distanc e nee d 29 Vehicle widt h 30 Vehicle length 31 Vehicle turning radius 32 Vehicle fr ont overhang, wheelbase, rear overhang, and ground clearan ce di me nsions


Exhibit 5-6. Visibility design concerns.

Concern or Issue Design Response Specific Procedure and/or Information

At driveway intersections with public roadways, have unobstructed lines-of-sight that provide adequate stopping sight distance.

Along high-type public roads, adequate intersection sight distance also should be provided. However, this may not always be practical in built-up areas.

Bicyclists, motorists in vehicles, and pedestrians need to see each other far enough in advance to avoid collisions.

Do not place anything in the border that blocks needed sight lines.

Refer to the latest edition of the AASHTO Green Book for the procedure to calculate the needed stopping sight distance or intersection sight distance.

To have time to react, drivers need to detect the driveway well in advance and be able to visually define its shape before entering or exiting.

Have driveway edge color contrast with the color of the abutting surface.

Have driveway pavement color contrast with the color of the roadway.

Consider illumination during darkness. For non-residential, place a monument

sign very close to the driveway intersection with the roadway.

Curbed driveways provide a clearer delineation of the driveway entry shape than “dust pan” designs do.


obstructions may also make it difficult for motorists on the roadway approaching the drivewayconnection to have an adequate preview of the driveway or vehicles in the driveway.

In Exhibit 5-5, the driver’s view from the street side provides clear definition of the edge.However, for the driver in the parking lot, the curb edge drop off is hidden, so, unable to detectthe dropoff, some vehicles drive over the curb. Practices similar to that shown in this exhibit,which create a continuous expanse of pavement and no distinction between the actual drivewayand the curb dropoff, should be avoided.

A similar problem can occur when a driveway that slopes downward from the roadway edgeis located on the high side of a superelevated roadway. Drivers in the roadway attempting to enterthe driveway may have difficulty determining where the driveway edges are. A designer may con-sider adjusting the driveway profile so that it rises slightly before descending, or adding delin-eators or soft landscaping to help drivers identify the driveway edges.

Exhibit 5-6 offers guidance for sight distance and conspicuity elements.

Exhibit 5-4. Placement of planter and utility polelimit the sight distance for a driveway exit.

Exhibit 5-5. Poorly defined edge leads to scrapes.

Driveways serving parking garages sometimes have restricted sight distance, especially wherethe vehicles exiting the garage cross the sidewalk abutting the edge of the garage structure.For guidance, refer to the AASHTO Guide for the Planning, Design, and Operation of PedestrianFacilities (5-2). Future studies could provide a better understanding of this.

Bicyclists

The driveway designer should recognize and accommodate interactions involving bicyclists,motor vehicles, and pedestrians. In this context, the main area of interaction involving bicy-clists is where they cross driveways. This may occur either when a bicyclist is riding in the publicroadway, crossing a driveway intersection, or on a bike path or other separate facility that crossesa driveway.

Although bicyclists often are not allowed to ride on sidewalks, there are exceptions. In somecommunities, younger children are allowed to ride on the sidewalk, except in the downtownarea (5-3). In some suburban and exurban areas, shared-use sidepaths are a common feature alongarterial roadways.

In general, bicyclists are more likely to be a consideration at driveways in urban and suburbanareas than they are in rural areas. Exhibit 5-7 lists some pertinent design principles.

Pedestrians and Pedestrians with Disabilities

In many environments, especially in built-up areas, pedestrians will be either crossing thedriveway or walking parallel to the driveway. Therefore, pedestrian needs must be consideredwhen designing a driveway. In some environments, pedestrian volumes will be practically nil,and in these situations pedestrian considerations may have less effect on design choices.

Where either existing sidewalks cross or future sidewalks will cross driveways, the drivewaydesigner must consider the horizontal alignment, the vertical alignment, and the cross slope ofthe pedestrian path. In the crossing area, the sidewalk design must conform to ADA requirements.Some sidewalk locations and some sidewalk and driveway designs more easily conform to ADArequirements than do others. Exhibit 5-8 lists some pedestrian design considerations.


Exhibit 5-7. Driveway design considerations related to bicyclists.


Provide horizontal and vertical alignment that provides an adequate advance view of the driveway intersection.

Bicyclists, motorists in vehicles, and pedestrians need to see each other far enough in advance to avoid collisions. Do not place anything in the border

that blocks needed sight lines.


Abrupt change in cross slope causes bicyclists to lose balance.

Where a bicycle path or other similar route crosses a driveway, do not have an abrupt change where the bike path cross slope meets the driveway grade.

Abrupt change in surface elevation causes bicyclists to lose control.

Where a bicyclist could turn into or turn out of a driveway, do not have an abrupt change in surface elevation that creates a bump for the bicyclist.

Relatively thin bicycle tires are vulnerable to openings in the surface.

Do not have any grate openings that a bicycle tire could drop into.


Exhibit 5-9 shows methods of aligning sidewalks at driveway crossings, so that the sidewalkdoes not exceed the ADA 2% cross slope requirement. (Some of these designs could just as easilyhave a radius return instead of a flared return.)

• With the setback or recessed sidewalk location, the driveway rises to the sidewalk elevationover the distance of the sidewalk setback from the curb.

• With the ramp or dipped sidewalk, the elevation of the sidewalk drops near the driveway cross-ing. The slope of the ramp on each side of the driveway is not to exceed 8%.

• With a sidewalk of sufficient width, a dustpan taper can be constructed adjacent to the curb,and still leave an adequate pedestrian route along the back edge of the sidewalk.

• The offset sidewalk is an adaptation of the wide sidewalk, with the sidewalk widened in thevicinity of the driveway to provide enough width for a dustpan and a pedestrian crossing.

The accompanying photo shows such an offset or jog in the sidewalk alignment, created so thepedestrian path will not have an abrupt change in elevation. If the normal sidewalk position hadbeen recessed or set back from the curb, with a grass strip between the sidewalk and the curb, thenthe pedestrian path could continue straight across the driveway, without the jog. Exhibit 5-10 showsan unacceptable treatment.

Exhibit 5-11 shows a driveway with very limited sight distance intersecting a sidewalk anda street.

A pedestrian crossing a driveway may be affected by factors such as the width of the drivewayto be crossed, the volume and the speed of vehicles using the driveway, the design of the side-walk crossing the driveway, the presence or absence of a pedestrian refuge island, or the presenceand location of a transit stop or other destination near the driveway.

A wider driveway increases a pedestrian’s time of exposure to conflicts with driveway vehicles.The width of the driveway crossing may be more of an issue for a child or older pedestrian whowalks slowly than for a wheelchair user.

A wider driveway may be more likely to seriously disorient a pedestrian with impaired vision.If pedestrians with impaired vision veer or are misaligned when they cross a driveway, unless

Exhibit 5-8. Driveway design considerations related to pedestrians.


Provide horizontal and vertical alignment that provides an adequate advance view of the driveway intersection.

Bicyclists, motorists in vehicles, and pedestrians need to see each other far enough in advance to avoid collisions. Do not place anything in the border

that blocks needed sight lines.


Also check sight for people of lower height, such as children or wheelchair users.

An excessive sidewalk cross slope (or driveway grade) adversely affects the crossing by pedestrians with vision impairments and those in wheelchairs.

ADA requirements specify a pedestrian travel path (called a Pedestrian Access Route or “PAR” in the Draft Public Rights-of-Way Accessibility Guidelines) with a cross slope that does not exceed 2%.

The PAR requirement applies not only to the crossing of the driveway, but also to the sidewalk connections.

The combination of 2% maximum cross slope and the different sidewalk location options affect the vertical alignment of the driveway in different ways.

Is there a suitable pedestrian route across the driveway?

The sidewalk alignment across the driveway should be straight and not have “steps” or other abrupt changes in vertical elevation.

Exhibit 5-9. Sidewalk-driveway treatments.


Exhibit 5-10. Unacceptable verticalcurb where sidewalk crosses driveway.

Exhibit 5-11. Driveway with very limited sight distance.

Exhibit 5-12. Bus stop locations near driveways.


another cue such as parallel traffic, a slope, or a guide strip is present, their veer can be expectedto be relatively constant during that crossing (5-4). As a result, if someone crossing a drivewayinitially has a 10-degree bearing error, they will likely continue in that direction. The wider thedriveway, the greater the chances are that pedestrians are farther from the sidewalk when theyreach the far side of the driveway. On a 20-foot crossing, a pedestrian with a slight veer might bejust outside the sidewalk area, and be able to easily locate the sidewalk by reaching out with a cane.On a 30- or 40-foot crossing with the same veer angle, a pedestrian may no longer be able to easilylocate the sidewalk on the far side of the driveway area. A cut-through median, a textured pedes-trian crossing, or a delineating guide strip across the driveway width might mitigate this situationon a wide driveway. Guidance strips are sometimes installed in the sidewalk to help guide pedes-trians with impaired vision across wide driveways. However, there is little current research on theability of pedestrians with impaired vision to use these guidance strips effectively (5-5).

Drivers may be more likely to yield to pedestrians if there is a wide landscape strip between thecurbline and the sidewalk or an auxiliary deceleration lane, so the vehicle turning into the drive-way can stop outside of the main travel lane of the roadway.

For pedestrians with impaired vision, it can be helpful if the driveway design discouragesvehicle encroachment onto the sidewalk area. Also, identifying the appropriate time to cross thedriveway can be a problem at a busy driveway—this problem may not be amenable to a geomet-ric remedy, except one that discourages high vehicular speeds.

Public Transit Facilities

The driveway designer should consider the location of transit routes and stops in the vicinityof the driveway. The following problems can arise when driveways and transit stops are too closeto each other:

• A stopped transit vehicle blocks the driveway.• Transit patrons block the driveway.• Standing transit patrons are uncomfortably close to driveway traffic.• Standing transit patrons block drivers’ lines-of-sight.

Exhibit 5-12 illustrates that a bus stop should be located to avoid blocking a driveway andset back a sufficient distance from the driveway to help ensure adequate sight distance. In manycases, a transit stop on the far side of the driveway connection with the roadway is preferable toone on the near side, because far-side bus stops do not interfere with vehicles turning right intodriveways and do not block the line of sight to the left of motorists exiting the driveway. Whenpossible, bus stops or driveways may need to be relocated to reduce conflicts with each other.Exhibit 5-13 provides guidance on the location and design of bus stops near driveways. Detailson bus stop location and design can be found in TCRP Report 19: Guidelines for the Location andDesign of Bus Stops (5-6).

Driveway Plan and Cross-Section Elements

The following sections discuss driveway plan and cross-section elements, such as the type of entry/exit geometry, the amount of flare or radius, the driveway width, and driveway channelization.

Driveway Width, Number of Lanes, and Connection Transition

This section discusses and presents recommendations for three plan view elements:

1. The normal width of the driveway throat, which does not include the normally found widen-ing or transition with a radius or a flare near the driveway intersection with the roadway;

2. The number of driveway lanes needed; and3. The shape and dimensions of the shape at the connection (throat entry/exit) transition.

The driveway width and the driveway connection transition are separate elements, but thedesign of each can affect the design of the other, so the discussion of these elements has beencombined.

Objectives for designing the driveway entry and exit geometry include the following:

1. Define the edge so it is visible for bicyclists, drivers, and pedestrians.2. Minimize the width of the driveway that bicyclists and pedestrians will need to cross.3. Design a shape that conforms to the path of the turning vehicle, which enables vehicles to

enter a driveway without encroaching into other lanes.4. Design to enable vehicles to enter the driveway without significantly impeding the upstream

flow of through traffic on the roadway.5. Provide adequate driveway capacity, including providing separate right- and left-turning exit

movements, when needed.6. Design for easy construction.

Exhibit 5-14 raises questions that address the design of the connection transition, andExhibit 5-15 shows the effects of inadequate geometry in this area. The geometry should notforce normal right entry or exit movements to cross over the driveway curb or edge, drive on


Exhibit 5-13. Driveway design considerations related to nearbybus stops.

Concern or Issue Design Response Specific Procedure and/or Information Provide adequate stopping

sight distance or intersection sight distance.

Provide separation from driveway edge to bus stop.

Do not block drivers’ lines-of-sight

Do not place bus shelters in a location that blocks needed sight lines.


Length of city transit bus: typical bus, about 40 ft. articulated bus, about 60 ft

Space so that the bus does not physically block the driveway

Provide distance from driveway edge to bus stop

Additional length along roadway needed for maneuvering:

where buses do not change lanes: 20 ft where a lane change is required: 50 ft

Patrons loading or unloading from transit vehicle do not occupy the driveway

Provide a pedestrian standing area separate from and removed from the driveway

Connect the bus loading area to the sidewalk

Pedestrian connection at least 5 feet wide, where possible

Exhibit 5-14. Design issues for a vehicle turning right into or from a driveway.

sidewalk

Does a vehicle turning intothe driveway encroach intothe adjacent lane?

Does vehicle encroachto the adjacent lane?

roadway

sidewalk

driveway

Does vehicle encroachon the curb or sidewalk?

Does vehicle encroachinto the adjacent lane?

Does vehicle encroachon the curb or sidewalk?

Does a vehicle turning outof the driveway encroach

into the adjacent lane?

Exhibit 5-15. Effects of inadequate driveway radii.

(a)

(b) (c)



the sidewalk, or swing wide so that the left side encroaches into adjacent lanes. But excessivewidth unnecessarily increases the distance across the driveway that bicyclists and pedestriansmust cross.

Interrelated Factors Affecting the Design

Various factors act in concert as a vehicle turns into or out of a driveway and as the vehiclecrosses a bicycle lane or sidewalk parallel to the roadway. The following interrelated elementscome into play as a driver turns into or out of a driveway intersection (5-7):

1. Visibility and conspicuity of the features that shape the driveway (e.g., opening, edges,markings);

2. Vehicle turning radius;3. Vehicle tracking width and offtracking characteristics;4. Intersection-corner treatment and treatment dimensions (e.g., radius or taper dimensions);5. Width of the lane from which the turn is made (includes offsets, edge flares);6. Width of the lane into which the turn is made (includes offsets, edge flares);7. Angle of the intersection;8. Cross slope of the pavement surface in the turn;9. Pavement surface condition (e.g., in extreme cases, a corrugated surface or pothole can

impart vertical acceleration to a turning vehicle, decreasing the available side friction);10. Turning speed;11. Driver’s tolerance of lateral acceleration; and12. Driver’s ability to perceive these elements.

The vertical profile also affects the driving experience.

Before selecting the dimensions of the connection transition, the designer should identifydesign vehicles for the particular driveway. Chapter 3 includes a discussion of design vehicle con-siderations. Where heavy vehicles may run over area behind the curb and damage surfaces suchas a sidewalk or a driveway median, the designer should consider a strengthened pavement sur-face for the affected area behind the curb.

Number of Lanes

A basic driveway design question is “how many driveway lanes should be provided?” Typically,driveways serving a single-family residence are one or two lanes wide, often reflecting the widthof the garage. Driveways serving farms and fields are typically one lane wide, although the widthof that lane is quite wide, reflecting the widths of farm machinery. In general, driveways servingcommercial and industrial sites should have at least two lanes (one-way driveways are an obviousexception), operating with one lane in each direction.

With increasing driveway volume, adding a second exit lane becomes highly desirable in orderto avoid excessively long queues and delay. Without two exit lanes, a motorist waiting for gapsin both traffic directions before turning left out of the driveway will unnecessarily block othermotorists in the exit queue who could otherwise turn right when there are gaps in the traffic fromthe left. However, the number of lanes exiting from the development and turning in one direc-tion must not exceed the number of available traffic lanes on the roadway in that direction. Forexample, for a driveway entering a two-lane two-way roadway, no more than one lane in eachdirection (a total of two exit lanes) should be allowed to exit the driveway. Generally, dual exit-lane driveways are desirable when the exit volume reaches the level that more than one vehiclewill want to exit the driveway within the time interval it takes one left-exiting vehicle to wait forand accept an adequate gap in roadway traffic, or when the driveway intersection with the pub-lic road is signalized. Exhibit 5-16 shows some of the more common commercial driveway con-figurations, excluding those for very high volumes.

Exhibit 5-17. Undesirable wide-open driveway.

Exhibit 5-16. Common choices for a range of commercial driveway designs.

Width for 1 entry and 2 exitlanes; separated by median

Width for 2 lanes. (Withouta marked center line, theremay be more encroachments.)

Width for 1 entry and 2 exitlanes; separated by yellowmarkings

yellow white white


If the driveway forms the fourth leg of an intersection, additional lanes may be needed. In suchcases, a configuration of three exit lanes (left turn, through, and right turn) and/or two entrylanes may be desirable. At what might be considered the high-volume end of the spectrum, sitessuch as major shopping centers and urban activity centers, even more lanes may be needed.

If there is a question of whether additional lanes are needed, an operational analysis of the inter-section between the driveway and the roadway, perhaps using calculations from the HighwayCapacity Manual (5-8), could be performed. Variables reflected in the operational analysis includethe volume on the main roadway, the volume and directional distribution of the driveway traffic,the number of adequate size gaps in through street traffic, and the form of traffic control at thedriveway/roadway intersection.

Driveway Width

The width of a driveway is its normal width, measured some distance back from its intersectionwith the roadway. It is not the width that includes widening near the intersecting roadway. Thewidth of a driveway is a function of the number of driveway lanes, the widths of those lanes,and the presence and width of a median.

The width of a driveway should reflect the needs of both vehicular and pedestrian traffic. Thecompeting goals of reducing vehicle delay by adding lanes and reducing pavement width to facil-itate pedestrian crossings need to be recognized.

Exhibit 5-17 shows a wide-open, undefined driveway across what appears to be the fullfrontage of the tract. These designs are particularly unfriendly to bicyclists and pedestrianscrossing the excessive driveway opening width. Because of the lack of lane definition, vehiclesenter and leave such sites in random positions and are more likely to cross paths. Such a designshould be avoided.

Exhibit 5-18 presents ranges of driveway widths and radii given in response to a survey oftransportation agencies.

Connection Transition Shape

For a driveway to intersect a public roadway, a break in the curb line or the edge of the road-way is required. This section discusses aspects of the transition that occur past the break, withinthe first few feet of the driveway. This transition may be designed with a perpendicular edge, rec-tangular apron, flare or taper, or curved radius. Exhibit 5-19 shows basic types of driveway con-nection transition geometries.

The driveway connection transition, where turning vehicles enter and exit the driveway, is usu-ally a critical design area, given that it is the location of potential interaction of entry and exitmovements (5-9). One of the key aspects of good driveway design is accommodating the entryand exit movements so that they do not encroach on one another or vehicles in other lanes (5-9).In some situations, this requires that the driveway radius be almost as large as the turning radiusof the selected design vehicle.

Except on low-volume, low-speed roadways, the curb radius or flare dimensions should bedesigned so that normal right-turn entry movements do not have to slow down to a near stop inthe through travel lanes on the roadway. The dimensions should also allow drivers to turn into orfrom a driveway without encroaching into conflicting lanes of traffic.

Where the roadway is curbed, the entry shape also acts in concert with the curb terminationtreatment at a driveway entry. Curbs may be terminated abruptly, by means of a drop-down curb,or by a return curb. Exhibit 5-20 shows curb termination treatments. (When the drop-down curbdesign accompanies a flare/taper transition edge shape, this is sometimes called a “dust pan.”)

Exhibit 5-21 compares the connection transition shape alternatives. The perpendicular edgeis the easiest to construct, but the least conspicuous to both motorists and pedestrians and doesnot conform to or help define the path of a turning vehicle. The rectangular apron is better than


Exhibit 5-18. Range of reported driveway widths and radii.

.Normally, Use This in Most Situations (ft)

Commercial (ft) Residential (ft)

Sm

alle

stre

port

ed

Ave

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Larg

est

repo

rted

Sm

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Width for 2-way: normal maximum (ft.) 24 34 40 35 40 46 12 23 30 Width for 2-way: normal minimum (ft.) 12 24 35 12 22 30 8 12 15 Entry-shape plan-view dimensions

for curved radius, maximum R (ft.) = 20 41 75 40 50 70 10 23 35 for curved radius, minimum R (ft.) = 3 16 25 15 21 30 3 11 15

NOTE: These values reflect survey responses from 1 local and 16 state transportation agencies.

Exhibit 5-19. Types of driveway connection transition geometry.

Curved radius

drivewaydriveway

drivewaydriveway

THESE ARE PLAN (TOP) VIEW DRAWINGS

drivewaydriveway

drivewaydriveway

drivewaydriveway

drivewaydriveway

drivewaydrivewaybackofcurb

backofcurb

backofcurb

backofcurb

roadway

driveway

Flare/Taper

driveway

roadway

driveway

Rectangular apron

driveway

roadway

driveway

Perpendicular edge

driveway

roadway

Exhibit 5-20. Methods to terminate the curb.

roadway

roadway curb

curb

driveway

Method to terminate the curb: RETURN CURB

driveway

roadway

curb

Method to terminate the curb:DROP-DOWN CURB

driveway

roadway

curb

Method to terminate the curb:ABRUPT END

driveway

(a) (b) (c)

Exhibit 5-21. Comparison of connection transition shape alternatives.

Design Objectives Perpendicular Edge

Rectangular Apron

Flare/Taper Curved Radius

Conforms to path of turning vehicle

worst (1) poor (2) better (3) best (4)

Definition of edge for motorists poor (2) worst (1) better (3) best (4) Definition of edge for pedestrians best (4) worst (1) poor (2) better (3) Ease of construction best (4) better (3) better (2) worst (1) Overall score best (12) NOTE: Cannot compare scores directly, because the importance or weight of each objective is not equal.


the perpendicular edge in terms of functionality, but slightly more difficult to construct. The useof both types should be limited to single-family or duplex residential units. The flared taper iseasier to build than the curved radius, but is less effective in terms of conspicuity and conform-ing to the path of a turning vehicle. Therefore, the use of the flared taper generally should be lim-ited to low intensity or medium intensity uses.

It has been stated that “Flared driveways are preferred because they are distinct from intersectiondelineations . . . “ (5-1, p.398); in other words, because they do not look like roadway intersections,motorists can distinguish between driveways and side streets. While this may be a benefit in a fewsituations, in many situations there is no benefit to be had from this distinction, and even if therewere, other aspects of driveway design will provide a visual difference for motorists to rely on.

As for curb termination treatments, an abrupt end is more likely to snag a vehicle tire, andtherefore is undesirable. A returned curb has a vertical face, which provides entry-edge definitionfor an approaching motorist.

A discussion of design treatments for sidewalks crossing the driveway in this area is inPedestrians and Pedestrians with Disabilities.

Exhibit 5-22 presents a table from the 2005 Florida Driveway Handbook (5-9, p.31), which wasderived from much older sources. The numerical values illustrate the inverse relationshipbetween entry radius and the width of the entry lane: as the size of the radius increases, less entrylane width is needed. Based on recent experience, these dimensions may be generous for manydrivers of passenger cars.

Connection Transition Design Suggestions

The preceding discussion of driveway transition shapes leads to the suggestions in Exhibit 5-23.

Driveway Width Design Suggestions

When establishing driveway widths and driveway opening treatment dimensions (e.g., size ofradius or flare), it is not uncommon to encounter opposing viewpoints; there may be conflict-ing objectives between the various driveway users, such as pedestrians and motorists, with some


Exhibit 5-22. Inverse relationship between entry radius and entry lane width.

Exhibit 5-23. Driveway transition shape design guidelines.

Category Description of Common Applications* Suggested Driveway Transition Shape Design (assuming curbed roadways in urban area, uncurbed in rural area)

STANDARD DRIVEWAYS

Very high intensity

Urban activity center, with almost constant driveway use during hours of operation. Typified by a driveway serving a post-1950 major shopping center or office complex. Not uncommon for such driveways to be signalized.

Design as a street intersection. Provide separate right- and left-turn lanes on approaches to public roadways.

Higher intensity

Medium-size office or retail, such as community shopping center, with frequent driveway use during hours of operation. Also includes land uses with extreme peaking patterns, such as public schools, worship assemblies, and employee parking lots.

Use curb radius design. Consider separate right- and left-turn lanes on approaches to public roads.

Medium intensity

Smaller office or retail, such as convenience stores, with occasional driveway use during hours of operation. Also includes some apartment complexes.

Curb radius design is preferred.

Lower intensity

Typical applications include single-family or duplex residential, other types with low use. May not apply to rural residential.

Use either the curb radius or the flare/taper design.



Building faces are close to the street. May have on-street parking or bus stops, a continuous sidewalk from the curb to faces of buildings, and higher pedestrian usage than in most other environments. Many situations will serve P-cars and some single-unit trucks.

Design will vary depending on location, land use, and traffic volumes.

Farm or ranch

May be a mixture of residential and industrial characteristics, used by a mix of design vehicles, such as P-car, single-unit truck, and agricultural equipment.

Design uncurbed radius or taper to accommodate farm/ranch vehicles.

Field Serves a field or other similar rural land area that is seldom trafficked. Higher-clearance P- vehicles or heavy vehicles are expected.

Design uncurbed radius or taper to accommodate farm/ranch vehicles.

Industrial Driveways frequently used by buses, tractor with semi-trailers, and other vehicles longer and wider than the design passenger car.

Design for trucks. The driveway may need a special design to accommodate the extra axles and longer wheelbase that will lead to much greater offtracking of vehicles entering the driveway.

* These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.

calling for smaller dimensions to make crossing the driveway easier for pedestrians, and otherswanting larger dimensions to facilitate motor vehicle ingress and egress.

An operational analysis of the intersection between the driveway and the roadway provides abasis for decisions regarding the number of driveway lanes. The connection transition and thedriveway width dimensions should complement each other to produce good driveway opera-tions. The driveway width and the curb radius can perform in concert, so to some degree onecan increase as the other decreases. In other words, a wide driveway can be used together with asmall radius or flare to achieve similar operations to a narrower driveway with a larger radius orflare. When only one vehicle is expected to be using the driveway at any given time, such as a res-idential driveway serving a two-car garage, the smaller radii are suitable with the greater widths.

Exhibit 5-24 offers guidelines for driveway width and radius. These dimensions do not con-sider the presence of an offset between the outer edge of the traveled way and the end of the

Exhibit 5-24. Driveway width and curb radius guidelines.

Category Description of Common Applications (Note: These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.)

Driveway Width Driveway Curb Radius (in ft)

Higher speedroad

Moderatespeedroad

Lowerspeedroad

STANDARD DRIVEWAYS Very high intensity


Many justify two lanes in, two to three lanes out. Refer to street design guides.

30–50 25–40 NA

Higherintensity

Medium-size office or retail (e.g., community shopping center) with frequent driveway use during hours of operation.

One entry lane, 12–13 ft wide

Two exit lanes, 11–13 ft wide.

25–40 20–35 NA

Medium intensity

Smaller office or retail, with occasional driveway use during hours of operation. Seldom more than one exiting vehicle at any time.

Two lanes, 24–26 ft total width

20–35 15–30 NA

Lowerintensity

Single-family or duplex residential, other types with low use, on lower speed/volume

A mix of design vehicles; somemay be very low volume.

roadways. May not apply to rural residential.

May be related to the width of the garage, or driveway parking.

Single lane: 9–12 ft Double: 16–20 ft

15–25 10–15 5–10

SPECIAL SITUATION DRIVEWAYSCentralbusinessdistrict

Building faces are close to the street.

Varies greatly, depending on use

NA 20–25 10–15

Farm or ranch;Field

Min. 16 ft, desirable 20 ft. Affected by widths of field machinery.

30–40 20–30 NA

Industrial Driveways are often used by large vehicles.

Minimum 26 ft 50–75 40–60 40–60

NOTES: These widths do not include space for a median or a parallel bike lane or sidewalk. Additional width may be needed if the driveway has a curved horizontal alignment. For a flare/taper design, use the radius as the dimension of the triangular legs. For industrial or other driveways frequented by heavy vehicles, consider either a simple curve with a taper

or a 3-centered curve design. For connection angles greatly different than 90 degrees, check the radius design with turning templates.

For connection corners at which a turn is prohibited, a very small radius is appropriate. Also see the section, Driveway Horizontal Alignment and Angle.

Driveways crossing an open ditch should have a minimum 2 ft shoulder on each side. (source: Statewide Urban Design and Specifications, Iowa State U., Ames, IA (October 21, 2008) p. 4.

If the roadway has a usable shoulder, a somewhat smaller radius may perform acceptably.



driveway, i.e., the driveway threshold. There are arguments for and against adjusting the radiuswhen an offset is present. Some agencies reduce the required radius when an offset is present,expecting turning vehicles to follow an effective radius that utilizes the space between the outeredge of the traveled way and the threshold. Arguments against this practice include an assump-tion that some drivers may not follow the imaginary effective radius, but instead try to followthe visible physical connection transition edges. Also, it is possible that, in future, the roadwaycross section width may be reallocated and the offset eliminated, resulting in an undersizedconnection transition.

One-Way Driveway Widths

Only a small fraction of driveways operate in a one-way mode. Information on which to baseguidance for the design of one-way driveways is limited and, as Exhibit 5-25 shows, current agen-cies’ standards differ considerably. Structured studies of one-way driveway design elementswould be helpful.

Throat Transition Design for Larger Vehicles

The offtracking of even-turning single-unit trucks can result in tires running over the curbor the sidewalk behind the curb. But if the designer accommodates turning trucks with a sim-ple radius design, this accommodation may create a very wide entry opening. To better accom-modate the wheel paths of turning trucks without paving such a wide area, refer to the AASHTOGreen Book’s (5-1, p. 583–621 ) discussion of designing simple curves with a taper and design-ing three-centered compound curves. Exhibit 5-26 dissects the geometry of a three-centeredcurve at a 90-deg. intersection.

Throat Width for Curved Driveways

If the driveway horizontal alignment is curved instead of straight, then additional drivewaywidth may be required to account for the effects of vehicle offtracking. Refer to the AASHTOGreen Book (5-1, p. 202–223) for procedures to determine how much additional width is needed.

Throat Transition Widening

Some driveways are constructed with a wider section close to the intersecting roadway, thenthe width tapers to a narrower section some distance back from the intersecting roadway. Twoof the reasons for doing this are to widen the driveway

1. To provide additional lanes at the intersection with the public roadway; and2. To accommodate the offtracking and swept paths of turning vehicles entering and exiting

driveways at the entry/exit area.

In either case, the designer will need to design a transition from the wider cross-section widthto the narrower cross-section width. Exhibit 5-27 shows a schematic of this concept.

A 6:1 taper would be adequate for an assumed design speed of 19 mph. Tapers of 8:1 to 12:1should be more than adequate for the typical driveway, excluding those that look and operatelike public roadways.

Exhibit 5-25. One-way driveway widths from selected states.

Agency Source Category Width for one-way Florida Driveway Handbook Urban 12 ft. minimum Missouri 940.16 (5/13/09) Driveway 20–30 ft New Jersey C-11 (6/20/07) Driveway 20–23 ft New York 608-03 (1/8/09) Minor Commercial 12–24 ft; 16 ft normal Utah 12.1.1601.10 Driveway 12–32 ft

Exhibit 5-26.Geometry of a sym-metrical 3-centeredcurve.

OffsetOffset

R1 R 2

R2

R1

Exhibit 5-28 shows an example 12:1 taper design. For a driveway having 12-ft-wide lanes, the15-ft radius provides a motorist with a 13.9 ft (12.0 + 1.9) opening at the point of tangency (PT),and the 20-ft radius produces a 13.5 ft (12.0 + 1.5) opening at the PT.

Channelization

Various types of channelization are sometimes incorporated into driveway designs. Theseinclude medians in the driveway, islands in the driveway, and channelization in the roadway atthe intersection with the driveway. The general design objectives for channelization are to

• Separate conflicting movements (including opposing directions of travel)• Control the angle of conflict• Reduce excessive pavement area• Regulate traffic and indicate proper use of driveway/intersection• Provide pedestrian refuges/protection• Provide for protection and storage of turning and crossing vehicles

Where channelization is desired but there is not sufficient space to accommodate the width ofa median or island, some agencies have installed channelizing devices such as tubular markers.These are discussed later in the section, Traffic Controls.

Channelization in the Roadway

Medians are sometimes labeled as being either non-restrictive or restrictive. A non-restrictivemedian is a median or painted centerline that does not provide a physical barrier between centertraffic turning lanes or traffic lanes traveling in opposite directions; examples include continuouscenter turn lanes and undivided highways. Restrictive medians physically separate vehicles travel-ing in opposite directions and restricts the movement of traffic across the median; (e.g., a concretebarrier or guard rail, a raised curb island, or a grassed or swaled median). Either type can be designedto provide some degree of separation between opposing traffic flows and provide space for left-turning vehicles out of the through lane.

Restrictive medians offer several safety benefits. Restrictive medians in the roadway providerefuge for pedestrians crossing the roadway. Some median openings allow all traffic movementsto be made. Other openings restrict left-turn and other movements across the median. Prohibitingmovements translates into fewer conflicts, greater safety, and more uniform travel speeds alongthe arterial. However, these benefits may be somewhat offset by the increased turning volumeswhere there are full-movement median openings. Restrictive medians are often used because, as

Exhibit 5-28. Example taper design.

X = 36’Q

X = 36’Qdriveway

12:1 taper Y = 3’

X = 36’Q

Offset

R

TA

N

TAN

2

R

PC

PT

road

way

Taper angle 4.764O 4.764O

/ 2 42.618O 42.618O

R 15 ft 20 ftTAN 13.80 ft 18.40 ft

Q 13.75 ft 18.34 ftOffset at PT 1.85 ft 1.47 ft


Exhibit 5-27. Taper to effectthroat transition widening.


indicated in Exhibit 5-29, more than two thirds of driveway crashes are related to left-turn entryor exit movements (5-10).

Exhibit 5-30 shows how restrictive medians may be used to eliminate some or all of the left-turn movements in and out of driveways. They may be channelized to allow for left-turns fromthe roadway, while prohibiting left-turns out. Alternatively, they may be channelized to allowfor left-turns from the driveway, while prohibiting left-turns in.

The following items are aspects of good median approach-island design (5-11):

• The approach nose should be offset from the approach lanes to minimize accidental impacts.• The shape of the island should be based on the turning path of the design vehicle and the island

function.• The length of the island should be related to approach speed and reflect available width, taper

design, and local constraints.• The width of the island should adequately serve its intended functions (e.g., access control,

pedestrian refuge, separation of conflicts, and shielding left-turn lanes).• Median islands should begin on tangent alignment and on upgrades or well past crest vertical

curves. It may be appropriate to extend a median island to avoid its introduction on a hori-zontal curve or within an area of limited sight distance.

Designs that prohibit some left-turn movements are often accompanied by an analysis oftraffic patterns that identifies alternate means of accomplishing a left turn, such as downstreamU-turns or other indirect means.

U-turns are used as an alternative to direct left turns to reduce conflicts and improve safety alongarterial roadways.They make it possible to eliminate left-turn movements into and out of driveways.They also make it possible to eliminate the need for certain traffic signals (or reduce the number

Exhibit 5-29. Driveway crash types related to maneuverand orientation.

Percent of TotalManeuver______Turn____Collision_______Driveway CrashesEntering Left Rear-end 26 Leaving Left Right-angle 24 Entering Left Head-on angle 15 Entering Right Rear-end 12 Leaving Right Right-angle 7 Leaving Right All other 8 Leaving Left All other 3 Entering Right All other 3 Entering Left All other 2

100

Source: Box, Public Safety Sys., 1969

Exhibit 5-30. Using restrictive medians to eliminate some left-turn movements.

Source: Transportation and Land Development, Second Edition.


of signal phases) that would not fit into a progression pattern (5-12). The median openings whereU-turns would be made need to be designed to accommodate the additional turning movements.

Requests for a median opening or opposition to closing a median opening may be based onan assumption that a direct left-turn egress maneuver is preferable to a right turn followed by aU-turn. However, observations show that drivers often make a right turn followed by a U-turnwhere the median opening design permits a direct left turn from a driveway (5-13). The addi-tional travel distance of turning right and then making a U-turn is offset by travel time savingsby not having to wait for a gap in both directions that is needed for direct, left-turn egress.

Medians in the Driveway

The benefits of restrictive medians in a roadway can also accrue when medians are installedin driveways. Medians in a driveway may be appropriate where one or more of the followingconditions exists:

• The driveway has two or more entrance lanes.• The driveway has two or more exit lanes.• There is a large pavement area that may confuse drivers.• The driveway operates as right-in/right-out, and this may be unclear to some drivers.• The driveway serves a high volume of traffic.• The driveway is or will be signalized.

A median in a driveway that separates the ingress and egress movements is appropriate forvery high-intensity driveways, where the median may provide refuge for pedestrians, separatethe opposing traffic flows, and channelize the traffic movements. Exhibit 5-31 provides guidancefor when a median in a driveway may be beneficial.

The presence of a median will make the overall length of the pedestrian crossing wider.However, this may be more than offset by the pedestrian refuge effect the median creates in themiddle of the driveway.

Where a driveway median is needed, there are minimum dimensions that apply to avoid hav-ing a median that is too short and narrow. There is also a possibility that if the median island istoo wide, drivers may mistake one driveway with a median for two separate driveways (5-9, p. 46).

Exhibit 5-31. Driveway median use recommendations.

Driveway Category

Description of Common Applications (Note: These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.)

Applicability of Median in Driveway

STANDARD DRIVEWAYS

Very high intensity


Applicable

Higher intensity

Medium - size office or retail, such as community shopping center, with frequent driveway use during hours of operation.

May be applicable

Medium intensity

Smaller office or retail, some apartment complexes.

Usually not applicable, but may be applicable for some wider driveways

Lower intensity


Not applicable



Building faces are close to the street. Usually not applicable, but may be applicable for some wider driveways

Farm or ranch; Field

A mix of design vehicles; some may be very low volume.

Usually not applicable

Industrial Driveways are often used by large vehicles.

Often not applicable, but may be applicable for some wider driveways

Exhibit 5-32. Driveway median design guidelines.

Aspect Suggested Design Rationale Length Minimum 40 ft, preferable 50 ft or more Need adequate length for

conspicuity,effectiveness.

Width Absolute minimum: 4 ft Minimum to provide pedestrian refuge: 6 ft Width for visibility of landscaping: 8 to 10 ft Maximum for a driveway divisional island (width of

the part that is unavailable for travel, i.e., not including turn lane widths): 12 to 16 ft.

Absolute minimum based on the Green Book.

Maximum based on potential for drivers to mistake one driveway with a median for two separate driveways.

Endtreatment

The 2004 AASHTO Green Book states that for a median island less than 10 ft wide, a semicircular end shape is adequate. For median island widths of 10 ft or more, a bullet nose end shape is suggested.

From observations, a bullet nose shape may be desirable for widths of less than 10 ft.

To fit the wheel path of a turning vehicle, per the 2004 Green Book (p.697).

Geometry of a bullet nose median-end shape

Radius of nose Median width

2

Stop line

Radius of nose

Half-bullet nose median-end shape

Exhibit 5-32 lists suggested minimum dimensions and presents two versions of bullet nose endgeometry. The half bullet nose provides a larger radius to accommodate the path and offtrack-ing of a vehicle nearing the end of its left turn. If the stop line and stopped position for vehiclesleaving the site is close to the median nose end, then a lesser radius may be adequate for that move-ment. Examine the turning paths of left-turning vehicles to ascertain what shape will suffice.

Islands in the Driveway

Driveway triangular islands (pork chops) can be constructed in the driveway entry throat atdriveway intersections with both divided and undivided roadways to

• Channelize right turns,• Discourage or prohibit one or both left turns, or• Provide refuge for pedestrians.

Exhibit 5-33 illustrates three different scenarios for using islands to discourage left turns.

A triangular island and an angled driveway can have some design objectives and features incommon. The objectives of either design can include facilitating right turns and discouragingleft turns. With both, the design can attempt to align vehicles at a skewed angle rather than per-pendicular to the public roadway.

Exhibit 5-34 shows two schematics for islands to channelize right turns exiting a driveway (5-13).In the (a) schematic, a flatter entry-angle combined with a larger radius may increase the speed atwhich right-turning vehicles leave the driveway. The flatter entry-angle requires the driver’s head toturn a greater number of degrees to the left to monitor oncoming traffic from the left. If the designevokes a subconscious association with a freeway entry ramp, it could theoretically give the driver afalse sense of a free entry into the through roadway. This arrangement has been criticized for being


unfriendly to pedestrians because of the relatively high speed of the right turn and the need fordrivers to scan a wide angle for pedestrians. The practice of placing the pedestrian crosswalk inthe middle of the curve, however, affords drivers an improved view of pedestrian crossings.

The second schematic (b) shows an alternative design that has several advantages. Motorists turn-ing right from a driveway can more easily see approaching through traffic. This design is more pedes-trian friendly because drivers have a better view of the sidewalk and the speeds are relatively slow.

The island area should be sufficiently large to command attention. Refer to the AASHTOGreen Book (5-1) to find the recommended minimum area and dimensions on a side. At loca-tions where there is a likelihood of traffic, especially large trucks, overrunning the island, theremay be a need for a mountable curb and structural pavement within the island.

Along roadways lacking a restrictive median, observations of traffic movements at triangularislands indicate it is not uncommon for drivers to make unusual maneuvers to circumvent theisland and make prohibited movements. It is challenging to identify a shape, size, radius, and otherfeatures that will effectively discourage drivers from circumventing the island. Florida DOT doesnot use driveway triangular islands on undivided roadways (5-9). Lakewood, Colorado, uses a40- by 18-ft island (360 sq ft) where all left turns are prohibited and a 20- by 18-ft (180 sq ft) islandwhere only the left turns leaving the driveway are prohibited. A design recommended for South

Exhibit 5-33. Using islands to discourage left-turn movements.


Exhibit 5-34. Comparing two right-turn island designs.




Dakota incorporated a long stem on the driveway-end of the triangular island to discouragewrong-way movements (see Exhibit 5-35). To better achieve the objective, triangular islandinstallations have been accompanied by the installation of a barrier median or by vertical pylons(traffic posts) along the middle of the public roadway.

Inappropriate Channelization

Although channelization can be beneficial, it can also be ineffective or inappropriate in somesituations. One does not have to look far to find examples (e.g., Exhibit 5-36) of questionable

Exhibit 5-35. Designproposed to discouragecircumventing triangular island.

Source: Dye Mgmt.Group, Review ofSDDOT’s Hwy.Access ControlProcess, Feb. 2000

CHANNELIZINGISLAND

roadway

driv

eway

Exhibit 5-36. Channelization with dubious benefits.

(a)

(b)

(c)

Exhibit 5-37. Sidewalk with visual, tactile, and geometriccues crossing a driveway.


island designs. The islands in the photos seem too small. It appears that the island in the upperphoto is aligned in a way that forces right-turning vehicles off of the pavement into the dirt. Onecan guess at the intent for islands in photos (b) and (c), but one wonders if they have any posi-tive effects on traffic, or if they are just obstacles and nuisances.

Visual and Tactile Cues

Providing visual and tactile cues that distinguish the sidewalk and define it separately fromother driveway areas can assist pedestrians having visual impairments to cross the driveway effi-ciently and safely. Texture, visual contrast, and slope differences are desirable.

Exhibit 5-37 shows a sidewalk crossing a driveway. The driveway has a distinct slope towardthe street. The slope between the street edge and the sidewalk edge is much greater than the slopeacross the sidewalk. The difference between the slopes may help pedestrians with vision impair-ments distinguish between the two areas and avoid accidentally veering into the street area asthey cross the driveway. There is also a color difference between the sidewalk and the drivewaythroat area and a slight texture difference between the sidewalk and asphalt which can be detectedby some pedestrians using a long cane.

Except for signalized driveways or a few other cases, the use of detectable warning surfaces, suchas truncated domes, is discouraged because overuse of detectable warnings surfaces may make itmore difficult for pedestrians with vision impairments to recognize streets and to maintain theirorientation (see Exhibit 5-38 for further discussion). Exhibit 5-39 shows a typical driveway con-struction plan for a detectable warning surface on a sidewalk at the edge of a signalized driveway.

Driveway Cross Slope

Where the driveway intersects the roadway, one side of a driveway will be higher than the otherside, unless the roadway that the driveway intersects is perfectly level. Proceeding from the trav-eled edge toward the private property, the designer can alter the relative difference in elevationbetween the two outer edges of the driveway by having different edge profile grades. Throughout

the length of the driveway, one edge may be higher than the other, or the center line may behigher than the edges, creating driveway cross slope. Where the driveway and sidewalk intersect,the driveway cross slope is the same as the sidewalk grade.

In the absence of information specifically developed for driveways, these guidelines have incor-porated cross slope recommendations from AASHTO. A minimum cross slope of 2% is recom-mended to provide surface runoff drainage. Where feasible, a maximum cross slope of 8% isrecommended in areas where snow or ice can occur.


Exhibit 5-38. When to use detectable warning surfaces.

Advisory R221 Detectable Warning Surfaces. Detectable warning surfaces arerequired where curb ramps, blended transitions, or landings provide a flushpedestrian connection to the street. Sidewalk crossings of residential drivewaysshould not generally be provided with detectable warnings, since the pedestrianright-of-way continues across most driveway aprons and overuse of detectablewarning surfaces should be avoided in the interests of message clarity. However,where commercial driveways are provided with traffic control devices or other-wise are permitted to operate like public streets, detectable warnings should beprovided at the junction between the pedestrian route and the street.

Source: Revised Draft Guidelines for Accessible Public Rights-of-Way, November 23, 2005

[http://www.access-board.gov/prowac/draft.htm#221 ]

Exhibit 5-39. Example of a detectable warning surface at edge of signalized driveway.

Driveway Horizontal Alignment and Angle

The alignment of a driveway near the connection with the public roadway affects trafficoperations and safety on both the driveway and the roadway. This section addresses drivewayhorizontal alignment and the angle of intersection with the roadway.

When there are driveways on opposite sides of the roadway from each other, the designershould check the alignment of through lanes, turning lanes, and medians on both the intersect-ing roadway and the driveway for potential operational and safety problems. The through lanesshould not be offset, but aligned. The review should include a check for whether the lane andmedian locations would create an offset that would obstruct sight distance (e.g., the sight distancebetween a vehicle turning left and an opposing through vehicle).

Angle of Intersection

Just as it is undesirable for two roadways to intersect at highly skewed angles, it is undesirablefor most driveways to intersect the roadway at a large skew. When a skew angle forces drivers todeal with a turning angle that is much less than or greater than 90 degrees, drivers will have greaterdifficulty turning their heads to scan the through roadway for an adequate gap, and more distanceand time is required to complete an acute angle turning movement. If the crossing is perpendi-cular to the driveway, it will also be more difficult for bicyclists and pedestrians about to cross adriveway to look over their shoulders to spot vehicles turning from the main roadway.

Research studies have concluded that the intersection angle should not be skewed from 90 degreesby more than 15 to 20 degrees (5-14 through 5-16). One-way driveways are an exception to this,and they have operated successfully with skew angle intersections with the roadway.

Exhibit 5-40 lists minimum allowable angles reported in a survey of transportation agencies.For two-way driveways, the average value allowed no more than about 20-deg. deviation from90 degrees.

Where the one-way driveway is intended to operate in a right-turn entry-only or a right-turnexit-only manner, there are tradeoffs. One theory is that a flatter angle (e.g., 45 degrees) makes itless likely that drivers will violate the right-turn-only intention or use the driveway in the wrongdirection. On the other hand, the flat exit requires that drivers entering the roadway turn theirheads much more than 90 degrees to see oncoming traffic from the left. Also, the greater the skewangle, the greater the crossing distance parallel to the roadway for bicyclists and pedestrians.

A review of state standards indicates that few allow an angle less than 60 degrees at one-waydriveways. Given strictly as an example, the following passage and drawing from the Ohio DOTmanual regulate the angle at which a driveway intersects the roadway:

803.21 Drive Intersection AngleNew drives should intersect the highway at an angle between 70° and 90°. However, in some cases, it may

be necessary to retain existing drive angles that vary from these desirable angles.


Exhibit 5-40. Range of reported allowable driveway intersection angles.

Normally, Use This in Most Situations

Commercial Residential

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alle

stre

port

ed

Ave

rage

Larg

est

r epo

rted

For 2-way drive, minimum angle with the roadway allowed (90O is right-angle) 60 68 90 60 69 75 60 70 90

For 1-way drive, minimum angle with the roadway allowed (90O is right-angle)

45 64 90 45 68 90 45 66 90



Exhibit 5-41 shows a guideline for angled, one-way driveways, from Ohio DOT design details.Exhibit 5-42 lists suggested minimum allowable intersection angles for driveways.

Driveway Horizontal Alignment

Past or back from the driveway connection transition area (the intersection with the roadway),the horizontal alignment (i.e., plan view) of a driveway should be straight, not curved. One rea-son for this is so the driver of a motor vehicle entering or leaving the driveway does not have theadded task of steering in a compound or reverse or multiple curves, which diverts more atten-tion from the task of monitoring crossing bicyclists, pedestrians, and vehicles. Another reason isthat a straight alignment makes it easier for drivers to position and align their vehicles as theyapproach the intersection and make turning maneuvers and not sideswipe other vehicles. A thirdreason is to avoid creating situations where the vehicle exiting the site is unintentionally posi-tioned at a skew angle to the roadway. Exhibit 5-43 recommends minimum lengths of straightapproaches in advance of the actual physical intersection of a driveway with a roadway.

Space for Bicyclists and Pedestrians

Motor vehicles are not the only form of traffic traveling perpendicular to the roadway to andfrom a traffic generator set back from the roadway. Bicyclists and pedestrians also make thesemovements at many locations and, in the absence of a separate facility, they may bike or walk inthe driveway.

Exhibit 5-44 shows a pedestrian on a gray, overcast day with light rain, forced to walk in thedriveway because of the lack of a sidewalk. At this particular location, the lack of a sidewalk con-tributes to occasional conflicts between vehicles and pedestrians.

Exhibit 5-41. Example of a skew-angle driveway.

Exhibit 5-42. Suggested driveway intersection angles with roadway.

Driveway Category Description of Common Applications*

Minimum Allowable Driveway Intersection Angle in Degrees

STANDARD DRIVEWAYS Very high intensity, Higher intensity, Medium intensity

70

Lower intensity Very infrequent use, such as single-family or duplex residential, on urban lower volume, lower speed roadways

60


CBD, Farm or ranch, Field, Industrial

70

One-way, for either right-turn entry-only or right-turn exit-only

Flat, acute angle may discourage wrong-way use

45 to 60


Exhibit 5-45 suggests situations where a separate facility parallel to the driveway may beneeded and where it may be acceptable for the bicyclist or pedestrian to share the driveway withmotor vehicles.

Driveway Edge and Border Treatments

A driveway edge should be clearly defined and visible to all users, so users can ascertain the lat-eral limits of motor vehicle operation. From observations such as in Exhibit 5-46, a vertical wallat the edge of a driveway causes drivers to shift their vehicles toward the center line. It is suggestedthat no vertical face (e.g., a retaining wall) be within 2 feet of the edge of the intended way for vehi-cle use. A wider offset must be provided if there will be a sidewalk parallel to the driveway.

For driveways with flat edges (i.e., no curb) in a fill, drivers will find it harder to determinewhere the edge is in rain, fog, or darkness. The designer should not place a sudden drop off at suchan edge. A relatively flat shoulder with a minimum width of 2 ft (after any rounding) is suggestedbefore the side slopes downward. Some property owners install reflectors or other similar devicesat the edge to help deal with this problem.

As shown in Exhibit 5-47a, the toe of a slope should not extend to the base of a drivewayor sidewalk edge, because runoff and erosion can lead to a mud-covered driveway or sidewalksurface. Exhibit 5-47b shows the toe of the slope recessed from the pavement edge, a methodwhich yields better results.

Edge Clearance from Fixed Objects

Fixed objects such as utility poles, fire hydrants, and drainage inlets should be set back fromthe edge of the driveway and from the edge of the roadway. Reasons for this include allowing

Exhibit 5-43. Suggested minimum lengths of straight driveway alignment.

Driveway Category

Description of Common Applications* Minimum Length of Straight Alignment on the Driveway Approach Adjacent to the Connection Transition with a Public Roadway

STANDARD DRIVEWAYS

Very high intensity

Urban activity center with almost constant driveway use during hours of operation.

75 ft (based on length of 3 P-car) Local requirements for tangent at a signalized intersection may apply.

Higherintensity


50 ft (based on length of 2 P-car)

Medium intensity

Smaller office or retail, some apartment complexes, with occasional driveway use during hours of operation.


Lowerintensity

Single-family or duplex residential, other types with very low use. May not apply to rural residential.



CBD, Farm or ranch,Field, Industrial

A mix of designvehicles.

Length equal to the design vehicle length, plus 5 ft

NOTE: The recommended lengths are based on orienting a likely number of vehicles to be present up to and through the driveway-roadway intersection. Further study to develop these values is needed. * These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.


Exhibit 5-44. Lack ofa sidewalk forces thepedestrian into thedriveway.


Exhibit 5-46. Effects of a vertical wall too close tothe driveway.

retaining wall is too close to driveway traffic; tohave a comfortable distance away from the wall,the driver moves over, across the centerline

Exhibit 5-45. Suggestions for separate facilities for bicyclists and pedestrians.

Driveway Category

Description of Common Applications* Need for a Facility Parallel to Driveway for Bicyclists or Pedestrians

STANDARD DRIVEWAYS

Very high intensity

Urban activity center with almost constant driveway use during hours of operation.

Bicycle – the need for separate lane or path depends on bicycle use in the area Pedestrian – often need sidewalk

Higherintensity


Bicycle – shared use may be adequate Pedestrian – may need sidewalk

Medium intensity

Smaller office or retail and some apartment complexes with occasional driveway use during hours of operation.

Bicycle – shared use usually adequate Pedestrian – may need sidewalk

Lowerintensity


Shared use is adequate


CBD Seldom applicable, because buildings are close to the street

Building faces are close to the street.

Farm or ranch,Field

Shared use is adequate Seldom used, very low volume.

Industrial Depends on the specific site plan and transportation modes used by the employees.

Driveways are often used by large vehicles.May have separate driveways for employeesand/or customers.



clearance for vehicle side mirrors and to account for the wheel and body paths of offtrackingturning vehicles. Exhibit 5-48 shows drainage inlets flanking a driveway, with both inlets show-ing what appears to be damage from turning vehicles.

Example design criteria such as Exhibit 5-49 suggest a clearance from vertically projecting fixedobjects (e.g., poles and fire hydrants) to the edge of the driveway of 5 feet or more. Objects suchas curb inlets should clear the paths of vehicles turning into and out of the driveway. Clear zonedesign practices affect the lateral placement of objects with respect to the edge of the traveled wayof a street or highway. The minimal urban clear zone may be inadequate in the immediate vicin-ity of the roadway-driveway intersection, and a larger dimension may better accommodate turn-ing vehicle offtracking.

The adequacy of any given driveway design can be checked with the turning templates of thedesign vehicle. The designer should also check to ensure that roadside objects do not becomeobstacles for other users (e.g., bicyclists and pedestrians).

Exhibit 5-47. Locating the toe of a slope near a driveway.

if the toe of the slope abuts the edges of the driveway or the sidewalk, then mudrunning down the slope can accumulate on the driveway and sidewalk, leaving amessy area

(a)

Toe of Slope

moving the toe of the slope back from theedges of the driveway and the sidewalkleaves space for run off to accumulate,making it less likely that mud will coverthe driveway or sidewalk

roadwayroadway

drive

way

drive

way (b)

Toe of Slope

Exhibit 5-48. Drainage inlets too close to the edgesof the driveway.

Exhibit 5-49. Driveway edge clearance from fixed objects.


Driveway Length

The following sections address the design of the following elements related to drivewaylength:

1. Minimum length of driveway to a barrier (e.g., garage door or gate)2. Minimum length of driveway paving3. Minimum length of the driveway throat or connection depth4. Accommodating the need to reverse vehicle direction (i.e., turn around) within the private site.

Drawings showing different facets of driveway length were provided at the beginning ofChapter 2 as well as on the following pages.

Driveways can be divided into two groups. One group operates in the manner typical of single-family suburban driveways, at which vehicles enter the driveway and then come to a stop andpark. The other group operates in a manner typical of commercial driveways—entering vehiclescontinue to move and proceed for some distance along the driveway, often into a parking lot.Some of the following controls are more likely to apply to the first category, while other controlsare more likely to apply to the second category.

Minimum Length of Driveway

For this section, the length of a driveway is the distance from where the driveway on oneend connects to the traveled way, to the other end where the driveway encounters some sortof barrier or terminates. This may be an intersecting circulation road within a site, the end ofthe pavement, a gate, a garage door, or other barrier that when in place, discourages or pre-vents a motor vehicle from proceeding. Even under the simplest of situations, many drivewayswill require a certain minimum length in order to avoid creating problems for one or moreuser groups.

Driveway Minimum Length Considerations

Problems can result when vehicles entering a driveway cannot proceed far enough into the driveway and parts of the vehicle then block the traveled way, bicycle lanes or paths, orsidewalks. Exhibit 5-50 shows vehicles parked in the driveways of townhouses constructed in the early 2000s. The rears of these vehicles partially block the sidewalk and lumber in thebed of the pickup truck in the foreground extends over the tailgate into the pedestrian path.This situation would be especially dangerous for a pedestrian with visual impairment usingthe sidewalk.

Unless a driveway is so short as to discourage its use for stopping or parking, the minimumlength between controlling features on each end of the driveway is the sum of the following threecomponents (see Exhibit 5-51):

1. Setback from the end toward the roadway to clear the outer edge of the traveled way, a bicyclelane or path, or a sidewalk

2. Length of the longest vehicle that typically would park there3. Clearance buffer from a gate, garage door, or other similar end-barrier

The buffer allows a person to walk between the end of the vehicle and the end-barrier. Thebuffer should also accommodate many drivers’ tendencies to shy away from a barrier, rather thanpulling close to it. It is hoped that the driver of a vehicle with a load that slightly hangs over therear will use the buffer to pull forward until the load clears the sidewalk.


Exhibit 5-50. Insufficient driveway length leads tovehicles blocking the sidewalk.

Exhibit 5-51. Minimum driveway length considerations.

++

driveway

sidewalkroadway

Where stopping or parking in driveway occurs,Minimum Driveway Length = sum of

1. setback toouter edge ofsidewalk, orother similarcontrol

+2. design vehicle length 3. buffer+

Driveway Minimum Length Design Suggestions

Given that the minimum length of the driveway is the sum of three values, two of which canvary greatly, prescribing a single or even a few values is of little benefit. Instead, it is recommendedthat the designer follow this series of steps:

1. Determine the longest vehicle type likely to use the driveway.2. Determine the length of that vehicle.3. Estimate a front buffer dimension. In the case of a smaller design vehicle (e.g., a P-car), esti-

mate 2 feet. For a larger design vehicle (e.g., a bus or large truck), select 3 feet. If the front bufferarea involves a gate that swings outward, there also should be an allowance for the gate.

4. Estimate a value for the rear clearance. Where a sidewalk exists, this is the distance from theedge of the traveled way to the far edge of the sidewalk. If no sidewalk exists, allow a mini-mum of 2 feet.

5. Sum these values to determine the minimum driveway length.

Research would be helpful to better define these dimensions (e.g., the actual buffer taken bythe drivers of various sizes of vehicles).

Minimum Length of Driveway Paving

If the driveway within the private property site is dirt or gravel, how far back from the edge ofthe traveled way to pave the driveway connection is an issue. A survey of transportation agenciesfound that practices varied among agencies, and no one practice predominated. Some agenciespave the driveway connection a fixed distance from the edge of the traveled way; others pave tothe right-of-way line.

The objectives of paving the connection to a gravel or dirt driveway some distance back from thetraveled way edge include (1) providing a more stable driveway surface “platform” from which toenter or exit the traveled way and (2) minimizing or eliminating the depositing of dirt, gravel, ormud onto the traveled way. Factors which can affect the extent to which debris from such a privatedriveway are deposited on the traveled way include

• The distance from the traveled way edge to the beginning of the gravel or dirt surface;• The grade of the driveway;• Surface drainage patterns, combined with the amount of precipitation; and• The volumes and types of traffic using the driveway.

Given the lack of consensus among agencies and the factors that affect the extent to whichdebris from unpaved driveways is deposited on the traveled way, guidance on this matter islimited to advising designers to pave driveway connections some distance back from the edgeof the traveled way. A designer may find guidance by observing the extent of the debris ema-nating from existing unpaved driveways in the vicinity of the driveway under consideration.

One specific suggestion is to pave a length at least as long as the length of the vehicles expectedto use the driveway, plus a clearance from the edge of the traveled way or sidewalk. This willencourage vehicles that pull off the road to clear the roadway and sidewalk. Another suggestionis that local governments require on-site paving or other mitigation actions to prevent debrisfrom washing onto the public roadway.

Minimum Length of Driveway Throat

The driveway throat length is the distance from the outer edge of the traveled way of the inter-secting roadway to the first point along the driveway at which there are conflicting vehicular traf-fic movements. Similar or related terms include the driveway connection depth, reservoir length,stacking distance, and storage length. Exhibit 5-52 illustrates a driveway throat.

Sources differ as to what point actually defines the internal (i.e., within the private site) endof the throat. Examples of these variations include “end of the driveway inside the land devel-opment” (5-17), “the parking lot served by a driveway” (5-18), and “the furthest end of thedriveway” (5-19). Given that an impetus for providing an adequate length for the drivewaythroat is related to allowing smooth traffic flow along the driveway between the street and on-site roadways or parking lots, the point at which conflicting traffic movements are encounteredwas selected to define the end of the driveway throat that lies within the site. So, implicit in thedefinition used herein is “non-conflicted” throat length.

Design Considerations

Providing an adequate driveway throat length or connection depth in which there are no con-flicting movements can help create smoother traffic flows in and near the driveway throat and avoidconflicts to which drivers may not have adequate time to react (which in turn may lead to colli-sions). As Exhibit 5-52 shows, an inadequate throat length can produce traffic situations thatadversely affect the flow of traffic on the public roadway. In this drawing, vehicles that have enteredthe driveway have formed a queue that blocks the sidewalk. Additional vehicles trying to enter thedriveway will not be able to proceed; therefore, they will stop in and block the public roadway too.

Exhibit 5-53 portrays a different situation that can affect the length of the right or entry side ofthe throat. All parking spaces should be far enough from the roadway, bike path, and/or sidewalk,so that vehicles backing out of parking stalls do not encroach into the projection of the sidewalkor bike path across the driveway, or into the roadway. Even if a vehicle backing out of a parkingspace into the driveway throat does not encroach into one of these areas, the backing vehicle will


Exhibit 5-52. Driveway throat.

parkinglot

THROAT LENGTH or

CONNECTION DEPTH

road

way

driveway

inte

rnal

(si

te)

road

sidewalk

Exhibit 5-53. Drivewayentry throat and parkingstalls.

road

way

VEHICLE BACKING OUTOF PARKING STALL

sidewalk


still block the driveway entry for the duration of the backing maneuver. In many situations, sucha blockage would have undesirable effects on traffic, so in those cases, parking should not beallowed in the driveway throat area.

Exhibit 5-54 shows yet another type of conflict, a speed hump installed in a driveway entry.Observations of traffic at this driveway revealed that as vehicles turned into the driveway, driverswere surprised by the need to rapidly decelerate over a short distance. Such a rapid decelerationincreases the driver’s exposure to being struck on the side by through vehicles and in the rear byfollowing vehicles. The process of slowing or stopping before turning into this driveway constitutesadequate traffic calming. When turning into a driveway while watching for conflicting bicyclists,pedestrians, and other vehicles, motorists should not be confronted with additional driving tasksuntil they have had time and distance to reorient themselves.

The following factors affect the distance needed to provide an adequate driveway throat length:

• The positions of bike paths and sidewalks.• The queuing or stacking space needed for exiting vehicles. If the exit is signalized, then suffi-

cient queuing length is needed to supply the green phase with vehicles proceeding at the sat-uration flow rate, accounting for lost time due to weaving on the driveway approach duringthe green phase.

• In cases of a multilane exit, the length needed for exiting vehicles to make weaving maneuversas they change lanes in the driveway.

• The distance needed to provide motorists entering the driveway with time to reorient them-selves and detect conflicting traffic movements from crossroads, parking spaces, bicycleroutes, or pedestrian paths they encounter.

• The queuing or stacking space needed for entering vehicles.• For a multilane entry, the length for entering vehicles to make weaving maneuvers as they

change lanes in the driveway.• For a one-way driveway, enough length to place Wrong Way/Do Not Enter signs so that the

intent is obvious to motorists.

Other considerations for the length of the throat found in the literature review include the

• Functional category of the intersecting roadway,• Type of driveway intersection traffic control (stop sign or signal) and traffic signal timing,• Type and intensity of land use activities served, and• Number of parking spaces (within the site) per exit lane.

Exhibit 5-54. Undesirable speed control in driveway throat.

Given that one of the underlying factors to consider is the driveway volume, both entering andexiting vehicles during the peak time period, it may be necessary to estimate likely driveway usagebefore designing the length of the driveway throat.

Design Suggestions

The throat length must be long enough to avoid internal site conflicts associated with cross-ing or weaving movements. It also must be adequate to avoid spillback onto the public road orinternal circulation system. There are different controls:

1. Designing sufficient length to react to conflicts,2. Designing sufficient length to accommodate traffic queues, and3. Designing sufficient length to accommodate weaving.

Different sources have developed different approaches for establishing minimum throatlengths. The following narrative presents the approaches from various sources.

Koepke and Levinson Throat Length. When more detailed, site-specific information isavailable, one could apply the recommendations by Koepke and Levinson in NCHRP Report 348(5-20). For signalized driveways, suggested on-site throat lengths (per lane) were based on theequation N = 2qr, where

N = number of cars to store,q = vehicles per hour per lane, andr = effective red time per cycle.

Alternative guidelines were cited based on the number of parking spaces per exit lane formulti-family, residential, retail, office, and industrial uses. The following suggested guidelineswere based on both sets of criteria:

• 50 feet for minor driveways that serve 50 to 100 apartments, less than 50,000 square feet ofretail, or a quality restaurant;

• 150 feet, with at least two exit lanes, for shopping centers of up to 700,000 square feet, andoffice complexes up to 500,000 square feet; and

• 200 feet or more, with at least two exit lanes, for larger commercial complexes.

Stover and Keopke Throat Length. In Transportation and Land Development, Stover andKoepke (5-13, p. 7–28) state that the exit condition controls the throat length for high-volumetraffic generators, while the entry condition controls the throat length for low-volume trafficgenerators. The exit side of a driveway should be designed to enable traffic to efficiently leave asite. The throat length and cross section are interrelated: the wider the cross section, the longerthe exit throat length needed to accommodate the associated weaving maneuvers. Exhibit 5-55presents the minimum throat length for stop-controlled and for signalized-access drives, basedon the number of egress or exit lanes.


Exhibit 5-55. Minimum throat length based on the type of control and number of lanes.

Type of Number of Exit Lanes Present Control 1 Exit Lane 2 Exit Lanes 3 Exit Lanes 4 Exit Lanes

STOP sign 30 to 50 ft 50 ft (2 cars) -- --

Signal -- 75 ft 200 ft 300 ft

NOTE: -- indicates no value givenSources: Transportation and Land Development, 2nd ed. (2002), p. 7-28 (5-13) and

Access Management Manual (2003), p. 184-185 (5-21)


The throat length for a one-way exit driveway needs to be sufficient to allow for DO NOTENTER or WRONG WAY signs to be installed to be effective in warning drivers turning fromthe roadway. This is related to liability, not queue length.

The throat length based on the entrance side of a driveway needs to minimize the potential forthe conflicts on the access drive from adversely affecting the intersecting roadway. Drivers enter-ing the site should clear the roadway intersection before encountering decision points and poten-tial conflicts along the driveway. Transportation and Land Development indicates the minimumthroat lengths for unsignalized access drives based on two driveway configurations – 1 entering/1 exiting lane and 1 entering/2 exiting lanes. For the 1 entering/1 exiting lane configuration,the minimum throat length is 75 feet to the first parking spaces on site or 30 feet to the firstintersection on site. For the 1 entering/2 exiting lane configuration, the minimum throat lengthis 75 feet to the first parking spaces on site or 50 feet to the first intersection on site. For high-volume traffic generators, it is the exit condition that governs the needed throat length.

Roseville Throat Length. The Roseville, California, design standards (5-22) present a detailedprocedure for estimating the needed length of the driveway throat. Agencies may apply the throatlength criteria from other sources to help establish a similar type of procedure.

The Roseville procedure is part of the traffic impact study process that applies to proposed proj-ects estimated to generate more than 50 PM peak-hour trip ends. The traffic study includes an eval-uation of the Minimum Required Throat Depth (MRTD) needed on-site for each access point fora proposed development. The MRTD requirement does not apply to single-family residential orduplex uses. The MRTD is measured from the back of the sidewalk to the first drive aisle or park-ing stall. The purpose of the MRTD is to allow enough stacking distance for egressing vehicles sothat the first drive aisle or parking stall is not blocked. This minimizes the possibility of incomingvehicles queuing out into the traveled way of the main street thereby creating a safety concern.

The MRTD is measured in car length increments of 25 feet and rounded up to the nearestmultiple of 25 feet. The City does not allow a MRTD of less than 25 feet for any project. Throatdepths greater than the calculated MRTD are encouraged. On-site parking is not permittedwithin the MRTD area.

The MRTD is a function of the length of the queue of vehicles waiting to exit the driveway. Thelength of this queue is a function of two variables: the number of vehicles desiring to egress dur-ing a given time period versus the number of vehicles that can enter the traffic stream of the mainroad during that same time period. If the calculated MRTD is physically or unreasonably too longfor the proposed development, then the traffic study can suggest ways to reduce the MRTD byeither decreasing the egress demand volume, or by increasing the movement capacity.

There are cases when an MRTD of 25 feet is acceptable, for example, when the first drive aisleis “one-way only” as shown in Exhibit 5-56. Another scenario where a MRTD of 25 feet is accept-able is when a raised center median is constructed in the driveway throat from the back of thesidewalk to the calculated MRTD distance. In this case, the nearest drive aisle can be two-way,but turning movements into and out of the drive aisle are restricted by the raised median.

Because of the different operations at signalized and unsignalized driveways, two differentmethodologies apply. At unsignalized project driveways, the MRTD is based on a series ofregression equations that the City uses to estimate maximum queue lengths at minor stop-controlled intersections. These equations apply the methodology presented in “Estimation ofMaximum Queue Lengths at Unsignalized Intersections” from the November 2001 ITE Journal.Exhibits 5-57 and 5-58 present the methodologies used for calculating the MRTD for variousunsignalized driveway conditions. Major street volumes are based on projected future trafficvolumes from the latest version of the citywide traffic model. Alternative methodologies for cal-


Exhibit 5-56. Roseville explanatory drawing.

Exhibit 5-57. Minimum required throat depth regression equations.

culating unsignalized MRTD lengths may be considered, but need to first be approved by thePublic Works Department prior to incorporation into traffic studies.

At signalized project driveways, MRTD lengths are a function of egressing traffic volumes, lanegeometrics, and traffic signal timing. Typically, signalized access locations will have more thanone approach lane for egressing vehicles; therefore, the MRTD is determined from the lane withthe longest queue. The MRTD is based on the Operational Analysis methodology contained in


the latest version of the Highway Capacity Manual or other methodology as approved by the City’sPublic Works Department. Major street volumes are based on projected future traffic volumesfrom the latest version of the citywide traffic model. For existing traffic signals, it is recommendedthat the consultant discuss likely signal timing parameters with City staff. There may be somerestrictions to signal timing parameters for existing signals because of progression and so forth.

The City also has provisions to help ensure that sufficient onsite storage is provided for drive-through service uses to ensure that vehicles will not queue into the public right-of-way.

The following definitions are for the terms used in the MRTD equations:

AppVol = hourly traffic volume divided by peak-hour factor (PHF) for subject movement

ConflVol = hourly traffic volume divided by PHF that conflicts with subject movement (referto the Highway Capacity Manual to identify movements that conflict with subject approach)

TS = a dummy variable with a value of 1 if a traffic signal is located on the major street withinone-quarter mile of the subject intersection and 0 otherwise

Lanes = number of through lanes occupied by conflicting traffic

Speed = posted speed limit on major street (in miles per hour)

RT% = Percentage of vehicles on shared left/through/right minor street approach that turn right

The following scenario employs several assumptions to illustrate another facet of principlesrelated to adequate throat length—minimizing traffic conflicts of the entry side of the drivewaythroat. Assuming a level, 90-degree entry, it was hypothesized that as drivers turn right into adriveway, the eventual 90-degree reorientation of drivers’ lines of sight is affected by factors suchas the following:

Exhibit 5-58. MRTD for right-turn-only movements.

1. Human factors. The span or width of drivers’ fields of vision and the degree to which driverscan turn their heads.

2. Vehicle factors. Limitations imposed by the structure of the vehicle (e.g., the position andwidth of the front windshield posts).

3. Operational factors. Informal observations suggest that drivers maneuvering vehicles into adriveway are not free to devote attention to conflicts in the driveway throat length ahead untilafter the entering vehicle has cleared any conflicts at the entry. This includes conflicts withpedestrians and “sideswipe conflicts” between the left front corner of the entering vehicle andthe left side of any vehicles exiting the driveway onto the public street.

Assume that drivers turning right into a driveway with a 25-ft radius at a speed of 15 mph or22.0 ft/s give full attention to the driveway ahead after completing 60 degrees of the turn. At thispoint, drivers have sufficiently squared-up their lines of sight and can detect a conflict (e.g., avehicle backing out of a parking stall or cross traffic within the site). The current AASHTO GreenBook guidelines (5-1, pp. 110–114) allow 2.5 seconds for a driver to react to an unexpected sit-uation ahead requiring the vehicle to stop; whether or not a conflict in the driveway ahead wouldconstitute an unexpected situation is arguable. For this illustration, assume that the driver has anarrow focus on the driveway ahead and requires only 1.0 second of perception-reaction timeand a deceleration rate of 11.2 ft/s2. This leads to the following calculations:

25 ft radius × tan (90° − 60°) = 14.4 ft25 ft tangent − 14.4 ft = 10.6 ft into driveway from edge of traveled way

Distance from driver to front bumper of vehicle: 6 ft

Distance into driveway + perception reaction distance + braking distance10.6 ft + 6 + [22.0 � 1.0] + [0.5 × 22.02 / 11.2] = 60 ft of entry throat length

Therefore, the designer would require a minimum of 60 feet of driveway connection depth fromthe outer edge of the traveled way to the first crossroad or other conflicting movements within thesite. If the first conflict encountered is with a vehicle backing out of a parking stall, then the positionof the rear bumper of the vehicle that has just backed out of the stall will also need to be taken intoaccount. Even with this non-conflicting connection depth, if a second vehicle is closely following thefirst and also turning right into the driveway, the driver of the second vehicle may not be able to reactand stop or the second vehicle may come to a stop with its rear still in the through roadway.

Other Throat Lengths. For a comparison, Exhibit 5-59 presents minimum throat lengthcriteria from two states, New Mexico and Florida. In both cases, the minimum requirementis 30 feet.


Exhibit 5-59. Throat length criteria from two states.

Source: New Mexico DOT, State Acc. Mgmt. Manual Ch. 8, Sec. 18, p. 91, Sept. 2001

FDOT Driveway Handbook, p. 54, Mar. 2005Source: Vergil Stover unpublished course notes

(b)(a)

Providing Onsite Turnaround Capability

It is often undesirable or unsafe for vehicles to perform a backing maneuver from a drivewayinto a public roadway. Therefore, most sites should be designed so that once entering the site, avehicle can be re-oriented and leave in a forward direction. This is highly desirable for all sitesexcept for single-family and duplex residences along lower volume, lower speed streets.

The type and design of needed turnaround facilities depend on the size of and types of activ-ities conducted on the site, the likely mix of vehicles, the building arrangements, and the circu-lation system for each site. Sometimes, turnaround needs can be accommodated by circulationon internal road systems or through parking areas. In other situations, a site needs a specific turn-around facility, such as a circle or other shape shown in Exhibit 5-60.

Internal Roadway Systems

Many developments, especially larger ones, have internal circulation systems that allow vehi-cles to enter the site and then, through a series of normal driving maneuvers, assume an orienta-tion that allows the vehicle to head out of the site.

Circulation Through Parking Lots

Somewhat similar to an internal roadway system but on a smaller scale, other sites have alayout that allows vehicles to circulate through the parking lot and leave the site in a forwarddirection. This type of turnaround works well with cars and smaller trucks, but may not beadequate for large trucks, unless greater maneuvering spaces are provided.

One form of this is “Loop Routing,” shown in Exhibit 5-60 (a and b). Where two parallel drive-ways enter a site, it is sometimes practical to turn around via an inverted “U” movement. For traf-fic driving on the right side, a counter clockwise movement has less internal traffic conflict, andleft turns around corners are easier for larger vehicles to negotiate than are right turns. Sometimes,a single driveway access point can be “split” via a loop road to provide the turnaround. This pat-tern is often seen at fast food restaurants, with the building located inside the loop.


Exhibit 5-60. Turnaround design schematics.

T-shape HammerheadT-shape HammerheadHammerhead

roadway roadway

U-shapedinternal circulationpattern

U-shapedinternalcirculationpattern

Offsetcul-de-sac

Cul-de-sac should have a reverse curve

Offsetcul-de-sac

Offsetcul-de-sac

Cul-de-sac should have a reverse curve

(a) (b)

(c)(d) (e) (f)


Specific Turnaround Facilities

When a site does not include features such as an internal roadway system or parking lot circu-lation that allow a driver to re-orient a vehicle and proceed from the site in a forward directiononto the public roadway, then specific turnaround facilities may be needed.

One common form is the cul-de-sac or circular turnaround. A circular turnaround can be cen-tered on the driveway or offset to one side. Circular turnarounds are generally preferable, althoughT-shaped and Y-shaped (hammerhead) turnarounds may be used. Exhibit 5-61 offers minimumdesirable circular turnaround dimensions (5-23).

With the T- and Y-shapes, vehicles turn left into the special roadway at the end of the driveway,back across the drive, and then proceed forward to turn left into the driveway. For passengercars, a 60- by 20-ft area is needed. Advantages of T- or Y-shaped turnarounds are that they havelower construction and maintenance costs and require less land than circular turnarounds.Because T- or Y-shaped turnarounds require all vehicles to make a back-up movement, theirapplication is limited to very low-volume driveways.

Driveway Vertical Alignment Elements

This section provides guidelines for designing the vertical alignment (or profile), which con-sists of grades and vertical curves. Designers should establish a vertical alignment that allowsvehicles to conveniently and expeditiously enter and exit the driveway. Designers should avoidprofiles that allow the underside of a vehicle to drag or hang-up. When establishing the verticalalignment of the driveway, the designer must consider limitations on the sidewalk cross slope toaccommodate pedestrians and pedestrians with disabilities. Also, designers should check theprofile to make sure it is not creating drainage problems.

Vertical Clearance

For many driveways, vertical clearance is not an issue. But where there are overhead struc-tures, utility lines, or vegetation, the designer should check that the vertical clearance is adequatefor the design vehicles.

Exhibit 5-61. Minimum desirable circular turnaround dimensions.

Vehicles Accommodated Radius to Face of Outer Curb (in feet)

Comments

Passenger cars only 30 Passenger cars, school buses, delivery trucks, emergency vehicles

42 A 45-ft radius would allow a central landscaped island with an inside curb radius of 25 feet and a 20-ft-wide turning roadway.

However, if a passenger vehicle is parked on the turning roadway, this geometry may not accommodate ambulance, fire truck, or solid waste vehicles.

Curb parking is permitted 50 Where a landing is provided at the base of the circle, a tangent section of 22 feet for each car should be provided.

Source: Kulash, Residential Streets, 3rd ed., Institute of Transportation Engineers, Washington, DC © 2001.Used by permission


Sidewalk Cross Slope (Driveway Grade)

When this guide was prepared, the ADA design requirements for accessibility related to pedes-trian facilities in the public right-of-way were still being developed. Although only draft accessi-bility guidelines and other technical assistance advisory documents from federal sources wereavailable, the ADA does and will continue to apply to sidewalks, curb ramps, and pedestriancrossings at driveways that are newly constructed or altered since January 26, 1992. The scop-ing provisions of these draft guidelines define where and to what degree accessibility within thepublic right-of-way is required and state the following:

R201.1 Scope. All newly designed and newly constructed facilities located in the public right-of-way shall

comply with these requirements. All altered portions of existing facilities located in the public right-of-way

shall comply with these requirements to the maximum extent feasible.

The ADA defines “facilities” very broadly and the US Department of Justice states in its ADATitle III regulations that this definition “ . . . includes both indoor and outdoor areas wherehuman-constructed improvements, structures, equipment, or property have been added to thenatural environment” (see 56 Fed. Register page 35550). The pedestrian crossing at a drivewayis a facility covered by the ADA and thus must be made accessible in new construction projectsand must be made accessible to the maximum extent feasible in alteration projects. The draftADA guidelines for public rights-of-way basically require that there be a continuous accessiblepedestrian route (i.e., PAR) leading up to and crossing each driveway. This will typically includethe following:

• The transition between the public sidewalk and the pedestrian crossing (marked or unmarked)at the driveway, which is usually achieved by means of an accessible curb ramp;

• The pedestrian cross walk pavement surface; and• Any island improvements within the driveway that pedestrians must traverse.

The pedestrian crossing at newly constructed driveways must offer a minimum 48-in.-wide routewith a cross slope no greater than 2 percent. Where the driveway is an alteration to existing improve-ments within the public right-of-way, the pedestrian crossing portion must offer a cross slope nosteeper than 2 percent to the maximum extent feasible, given existing site-related constraints.

Site-related constraints that may prohibit strict compliance with the ADA maximum 2 percentcross slope specifications include severely limited right-of-way or sidewalk width in which to nego-tiate the vertical rise between the roadway elevation and the parking area, or steep existing gradeson an adjoining, densely developed property that the driveway serves. Engineering judgment playsa key role during the design of driveway alteration projects where full accessibility is not beingoffered and that judgment may be challenged under ADA by experts analyzing every detail of thedesign and site factors that may or may not be found to justify any alleged access barriers createdby the design.

Exhibit 5-62 shows a driveway grade rising quickly from the gutter line, creating an excessiveand unacceptable cross slope for the pedestrian path.

The five illustrations constituting Exhibit 5-63 (taken from the recommendations of the USAccess Board’s Public Rights-of-Way Accessibility Advisory Committee published in “Buildinga True Community”) demonstrate driveway design options that comply with the accessibilityspecifications in the draft ADA guidelines.

• Option A, Ramp Sidewalk, shows the sidewalk simply ramping down at each side of the drive-way with a maximum 2 percent or 1:48 cross slope along the pedestrian crossing.

• Option B, Apron Offset Sidewalk, shows a directional offset in the sidewalk to avoid the steepcross slope that would otherwise be created by crossing the driveway apron on the steeply slop-ing portion.

• Option C, Gutter Bridgeplate, shows the whole width of the sidewalk having a limited crossslope and employs a bridgeplate over the gutter at a rolled curb condition to limit the likeli-hood of vehicles bottoming out.

• Option D, Wide Sidewalk, uses the rear most 48 inches of the driveway apron to cross the drivewithout a cross slope steeper than 2 percent.

• Option E, Setback Sidewalk, shows how a more traditional returned curb style driveway aproncan be installed between the gutter and the street side of the sidewalk which adjoins a land-scaped green space.

At pedestrian crossings in driveways of more developed commercial sites where the drive-way more closely resembles a street, the design and construction of the pedestrian crossingarea between the curb ramps is subject to the same 2% maximum cross slope that other drive-ways are subject to. In Exhibit 5-64, the driveway leading into a regional shopping mall lookssimilar to a roadway intersection; the curb ramps and accessible pedestrian crossings shouldbe constructed with a maximum 2% cross slope.


Exhibit 5-62. Example of driveway grade creatingunacceptable sidewalk cross slope.

Exhibit 5-63. Examples of driveways thatcomply with accessibility specifications.

Exhibit 5-64. Driveway entrance at regional mall.


Driveway-sidewalk crossing transitions call for special attention. Of particular concern arethe multi-dimensional tapers that arise from dust-pan and similar flared treatments. The 2004AASHTO pedestrian design guide points out that “side flares and cross slopes at drivewayaprons may cause a drive wheel, caster, or leg tip to lose contact with the surface” (5-2, p. 61–62).Therefore, such flares should not be used unless there is another suitable PAR, such as mightbe provided by a wide sidewalk.

Driveway Grade (Sidewalk Cross Slope), Change of Grade, and Vertical Alignment

Three types of control for the design of the driveway profile are physical, operational, anddrainage:

• Physical controls call for a design that maintains enough clearance so the underside of a vehi-cle does not drag on the roadway or driveway surface. This control is necessary for all drive-ways, even one connecting to an alley. Because of the changes in vertical profile grade oftenfound at driveway entrances, these locations are among the more vulnerable to hang ups whenthe undercarriage of the vehicle comes into contact with or drags the pavement surface.

• Operational controls dictate a vertical alignment for the driveway that allows a convenientand safe entry with minimal conflicts. To achieve this, the changes of gradient must not be tooabrupt. This is especially important on driveways that intersect higher volume or higher speedroadways. Operational problems may arise from certain combinations of vertical profiles andvehicles. One problem is vehicle-occupant discomfort due to poor vertical alignment such asbumps, steep grades, and abrupt changes in grade. In extreme cases, there may be restrictedsight distance, which affect safety adversely. In addition, excessive differences in speed betweenthrough vehicles and vehicles turning into or out of the driveway, because of the vertical pro-file, can also increase vehicles’ exposure to crashes.

• Drainage, requires a profile that does not create undesirable drainage patterns. It may be unac-ceptable for surface runoff in the gutter to flow into the driveway opening and onto privateproperty.

Physical Vehicle Ground Clearance Control

As Exhibit 5-65 shows, the underside of a vehicle entering or exiting a driveway can drag oneither a crest or a sag alignment with an abrupt change of grade. Any excessive grade changebetween the cross slope of the roadway and the driveway grade, between the driveway grade andan intersecting sidewalk, or between successive driveway grades can cause a vehicle to drag (seeExhibit 5-66). Vehicles with low ground clearance and a long wheelbase or overhang can evenbecome lodged (also referred to as “hung up” or “high-centered”) on alignments with sharp gradechanges. At best, hang-ups result in some vehicular delay and minor damage to the undercarriageof the vehicle and to the pavement surface. At worst, a crash can occur.

Exhibit 5-65. Geometry of ground clearance dragging.

WB=wheelbase

(b)(a)

OHF= front overhang OHR= rear overhang

WB=wheelbase OHF= front overhang OHR= rear overhang

roadwaycurb

roadwayOHR

SAG: Have problem if axle-to-bumperunderclearance is inadequate. For symmetricalsag, is critical when one axle is at distance WBfrom the low point.

roadwaycurb

CREST: Have problem if axle-to-axleunderclearance is inadequate. For symmetricalcrest, is critical when axles are a distance WB/2from the high point.

roadwaycurb

WB/2

driveway

WB/2roadwayroadway

curb

roadwayOHR WB

curbroadwayroadwayOHFOHR

driveway

To design the vertical alignment elements, the designer needs to determine an appropriatedesign vehicle. As previously discussed, several types of long-wheelbase, low-ground-clearancevehicles can be expected to use some driveways, including articulated beverage trucks, car carri-ers, and passenger car-trailer combinations. The design vehicle for vertical alignment may be dif-ferent from the design vehicle used to design the horizontal alignment (e.g., turning radii). Thedesigner also needs to have a general understanding of the shape of the vertical profile to be nego-tiated by the design vehicle. This includes, for example, the roadway cross slope, the drivewaygrade line, and other controls (e.g., locations and elevations of intersecting sidewalks).

Using reasonable care in selecting a design vehicle and designing the vertical elements toaccommodate that vehicle will not completely preclude hang-ups, dragging, or other operationalproblems from occurring. A vehicle longer and/or lower than the design vehicle may enter adriveway and encounter problems. To meet the needs of shippers, commercial vehicle manufac-turers continue to introduce longer and/or lower vehicles and new vehicle configurations thatwill require periodic updating of the list of design vehicles. Similarly, as property changes hands oras redevelopment occurs, the nature of the land use served by the driveway may change over time.A different class of vehicle than originally intended may use the driveway. Although this is beyondthe control of the designer, it offers an explanation of why hang-ups may happen at locations wherethey formerly did not occur and represents an issue to be addressed in the permitting process.

Also, a vehicle for which the vertical elements have been appropriately designed may encounterproblems at a particular driveway. This could be due to vehicle loading condition (e.g., an over-loaded vehicle) that reduces actual ground clearance to something below the design value. Thevertical profile is subject to changes over time. For example, the roadway may be milled or resur-faced such that its elevations and cross slopes change. In addition, the roadway and/or driveway(and associated features such as sidewalks) may deform over time due to applied loads, the effectsof weather, or construction deficiencies. As mentioned above, the vertical profile(s) used in designshould be that reasonably expected to be used by the design vehicle. The possibility always existsthat a design vehicle will follow an unusual or out-of-the-ordinary path in negotiating the drive-way such that hang-up or dragging could result.

Exhibit 5-67 shows maximum uphill and downhill grades, as reported by transportationagencies in a survey.


Exhibit 5-66. Driveway with multiple scrapes fromunderside dragging.


Maximum allowable grade, by itself, is not a sufficient control. What matters is the differencebetween successive grades, or the change of grade. The change of grade is what creates the crestsand sags that cause the underside of a vehicle to drag. Although perhaps not widely recognized,guidance on vertical geometry applicable to driveways has been available for some time. The sec-tion on railroad-highway grade crossing design in AASHTO’s policy on geometric design (5-1)provides recommendations on designing the vertical profile at grade crossings. AASHTO rec-ommends that the crossing surface be in the same plane as the top of rails for a distance of 2 feetoutside of the rails, and that the surface of the roadway be not more than 3 inches higher or lowerthan the top of the nearest rail at a point 30 feet from the rail, unless track superelevation dic-tates otherwise.

Similarly, a 1987 ITE guideline for driveway design discussed vertical alignment. Eck and Kang(5-24) used a vehicle with a 36-ft wheelbase and 5 inches of ground clearance to analyze a maxi-mum grade change of 3 percent (for low-volume driveways on major or collector streets). This“design vehicle” had problems with the aforementioned geometry, suggesting that the ITE drive-way design recommendations did not accommodate low-clearance vehicles. A similar statementcan be made about the AASHTO standard railroad-highway grade crossing since French,Clawson, and Eck (5-25) found that car carrier trailers would hang-up on this crossing. Thus,additional research was conducted to develop driveway vertical alignment guidelines to accom-modate selected design vehicles.

Operational Control

A research team made measurements at 31 driveways with visible scrapes from the undersidesof vehicles, and then measured speeds and elapsed travel times for over 1500 vehicles observedturning right or left into a number of driveways. The speed and elapsed time studies were con-ducted at commercial driveways on built-up suburban (but not CBD) arterial multilane roadwayswith posted speeds of 40 and 45 mph. All of the roadways had either a raised median or a TWLTL.These data were collected at driveways with right-turn entry radii ranging from 13 to 19.5 feet,and an entry lane width of about 13 feet.

Very few vehicles about to enter a driveway exceeded 20 mph at the locations at which speedswere measured. After crossing the driveway threshold, average speeds for vehicles turning left intothe driveway were around 10 mph. Vehicles that had turned right into the driveways were slightlyslower, with average speeds around 7 mph. The speeds of vehicles entering driveways withbreakover sag grades up to 10.5% were close to the speeds of vehicles entering flatter driveways.Scrapes on the pavement surface, presumably from the undersides of vehicles, began to be com-mon with a sag breakover of around 10 percent, and a crest breakover of about 11 percent.

Exhibit 5-67. Reported steepest allowed driveway grades.

Normally, Use This in Most Situations

Commercial Residential

Sm

alle

stre

port

ed

Ave

rage

Larg

est

repo

rted

Sm

alle

stre

port

ed

Ave

rage

Larg

est

repo

rted

Sm

alle

stre

port

ed

Ave

rage

Larg

est

repo

rted

Grade: maximum (+) uphill from road allowed 2.6 9.7 15 5 7.5 10 6 11 15

Grade: maximum (-) downhill from road allowed

-5 -9.4 -15 -5 -7.8 -10 -6 -11.0 -15


+ uphill+ uphill

driveway

- downhill

roadway

roadway

- downhilldriveway


Exhibit 5-68. Driveway vertical profile guidelines.

Category Description of Common Applications*

Vertical Profile Guidelines

Suggestion Rationale

STANDARD DRIVEWAYS

Very high intensity


Refer to roadway design guidelines.

These driveways are often built to the standards of and resemble public roads and streets.

FOR BOTH

Higher intensity

Medium-size office or retail, such as community shopping center, with frequent driveway use during hours of operation.

Limit the maximum driveway grade to +8% (except where a lesser grade is required, such as when crossing a sidewalk), and the maximum sag breakover without a vertical curve between the roadway cross slope and an uphill driveway grade to 9%.

Limit the driveway profile maximum grade change without a vertical curve for: a crest to 10% and a sagto 9%.

From observations of vehicles entering driveways with radii up to 20 ft and comparisons of Flatter (1.5-5%) and Moderate (6-9%) grades revealed (1) little differencebetween speeds and travel times of vehicles turning right; and (2) only slight differences betweenspeeds and travel times of vehiclesturning left.

From measurements of 31 driveways with scrape marks, underside dragging became a problem at a crest of about 11%, and at a sag of about 10%.

AND

Medium intensity

Smaller office, retail, or other sites with occasional driveway use during hours of operation.

Apartment complexes May limit the sag to 7%. Due to trailers.

Lower intensity

Single family or duplex residential, other types with very low use. May not apply to rural residential.

Limit the driveway profile maximum grade change without a vertical curve for: a crest to 10% and a sag to 9%.

From measurements of 31 driveways with scrape marks, underside dragging became a problem at a crest of about 11%, and at a sag of about 10%.


CBD Refer to the guidelines above for “Higher intensity” and “Medium intensity.”

Building faces areclose to the street.

Farm or ranch; Field

Limit the driveway profile maximum grade change without a vertical curve for:a crest to 10% and a sag to 7%.

A mix of designvehicles; some maybe very low volume.

These driveways should accommodate trailers.

Industrial Varies, depending on types of vehicles. If low-boy trailers are expected, then limit crest

Driveways are oftenused by large vehicles.

breakover grades without a vertical curve to 3.5%.

Other Motels Limit the driveway profile maximum grade change without a vertical curve for:a crest to 10% and a sag to 7%.

Travelers pulling a trailer may stay at a motel; therefore, motel driveways should accommodate trailers.

NOTES: Additional information on which to assess ground clearance is in Chp 3. The sag clearance for trailers is based on Eck’s evaluation; truck+trailer clearances will vary. * These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.

The study led to the suggestions following in Exhibit 5-68. Except where noted, these guide-lines are based on observations of passenger vehicles (P-vehicle).

Where low-clearance vehicles are expected to traverse crest curves, refer to Exhibit 5-69 devel-oped by Eck and Kang (5-26) that suggests vertical curve lengths for various breakover angles(i.e., algebraic difference in grades).

Drainage Control

Surface runoff from the roadway should not inundate the sidewalk or spill over onto privateproperty. It is also undesirable for the depth of flow to cover the driveway, making it difficult formotorists to determine were the edges of the driveway are.


There are a number of possible design scenarios, based on combinations of curbed oruncurbed roadways with driveway profiles that extend uphill or downhill from the connectionwith the roadway. Among the tools to combat surface runoff are driveway profile, driveway crossslope, drainage inlets near the driveway area, and drainage grates in the driveway. Exhibit 5-70shows how profile design can be used to prevent water in the gutter from flowing onto privateproperty.

Roadway-Driveway Threshold Treatment

The threshold is the edge or line where the roadway and the driveway join or touch. This line isoften at the curb edge. Design concerns in this area include ease of travel for users (e.g., bicyclesand motor vehicles), ease of construction, and, in cases where the roadway has a curb and gut-ter, confining drainage to the gutter line. Exhibit 5-71 shows four common driveway thresholdtreatments where the roadway has curbs: rolled curb, vertical lip, counterslope, and continuous.Exhibit 5-72 suggests design guidelines for driveway threshold treatments.

The Continuous design is the preferred method. Except for single-family or duplex access onlower volume, lower speed residential streets, designers should avoid designs that create a bumpat the threshold. Even in the single-family context, consider that a vertical discontinuity can be animpediment for bicyclists as well as pedestrians with disabilities (especially using wheelchairs).Vertical lip design is another topic needing additional research to assess the ability of other treat-ments to address drainage, to assess the detrimental effects of a pronounced lip, and to determinewhether a low lip, perhaps on the magnitude of 1⁄2 inch, has any detrimental effects on users.

For any type of treatment, the curb and gutter should not be broken off to leave a ragged edge,but should be cut with a saw and cleanly removed.

Exhibit 5-73 shows a gutter treatment used in some jurisdictions. The gutter cross slope is sig-nificantly greater than that of the adjacent traveled lanes. This treatment is believed to improvedrainage; however, it also increases the profile breakover angle which motorists entering and

Exhibit 5-69. Minimum length of Type-II crest verticalcurve to accommodate low-clearance vehicle.

Algebraic Difference (%) Curve Length ft(m) 1 4 (1.2) 2 8 (2.4) 3 12 (3.7) 4 16 (4.9) 5 20 (6.1) 6 24 (7.3) 7 28 (8.5) 8 32 (9.8) 9 35 (10.7) 10 39 (11.9)

Exhibit 5-70. Confining surface runoff flow.

Roadway with curb: Setting the drivewayprofile with a crest vertical curve to slopedown to the gutter, to confine the flow.

ground

ground

drivewaydriveway

roadway

Roadway without curb: The driveway will

(b)(a)

need a crown, cross slope, or a grate inthe sag to provide surface drainage.

roadwayshoulder


Exhibit 5-71. Driveway threshold treatment types.

near-vertical lip at the gutterline

STREET CROSS SECTION -DRIVEWAY PROFILE VIEW

VERTICAL LIP

curb

curb shape does not changeat a driveway


ROLLED CURB

street

street street

streetcurb

curb curb

driveway(may slopeup or down)

drivewaydriveway

(may slopeup or down)

(may slopeup or down)

driveway(may slopeup or down)

(b)(a)

(d)(c)

incline (steeper than drivewaygrade) behind the gutter line


COUNTERSLOPE

no abrupt vertical component;driveway grade connects atgutter line


CONTINUOUS

Exhibit 5-72. Driveway threshold treatment guidelines.

Method Advantages and Disadvantages Comments Rolled curb Easiest threshold to construct, because the

existing curb is not modified or removed. Confines the gutter flow, since the existing

curb remains intact. Vehicles entering or exiting the driveway

experience a jolt while crossing a curb of typical height.

This method is generally unsuitable.

It may be acceptable for single-family or duplex access on lower volume, lower speed residential streets.

Vertical lip Construction requires curb modification or removal.

Can confine very low flows in the gutter and reduce the spread of the gutter flow.

Bump created by the vertical lip is a minor impediment to automobile movements and a more significant problem for turning bicyclists (i.e., bicycle tire strikes the face at a skew angle).

Is often constructed by forming the threshold with lumber that leaves a vertical face or lip of 1 to 2 inches at the threshold.

Counterslope Construction requires curb modification or removal.

Can confine very low flows in the gutter and reduce the spread of the gutter flow.

Less abrupt to cross, but can still be disruptive to automobiles and bicycles.

The proportion and amount ofrise and run affect the degreeof disruption to automobilesand bicycles.

ContinuousOR Smooth

Construction requires curb modification or removal.

Is more bicycle- and automobile-friendly. If the driveway immediately slopes downward

With this method, the profile slopes continuously but not abruptly upward from the gutter line. Thus the drainageobjective can be suitably achieved by means that do not create problematic bumpsfor bicyclists or drivers.

from the gutter line, this does not confine thedrainage as well.


exiting the driveway have to negotiate. Many scrape marks on the driveway surface from thedragging of vehicles bumpers are clearly visible. More study should be done on this type of designto weigh any drainage benefits against impediments to traffic flow.

Vertical Alignment Examples

The following examples apply some of the guidelines for designing the vertical alignment ofdriveways. Exhibit 5-74 shows the driveway profile rising from the gutter line up to the sidewalk,then flattening at the sidewalk before falling as the driveway continues onto the private property.This type of design will confine normal depths of water in the gutter and not allow water to flowon to private property and down the driveway.

Exhibit 5-75 shows the suggested values for driveways at which the P-vehicle is the design con-trol. If the near edge of the sidewalk is 5.5 feet from the face-of-curb or gutter line, and the drive-way is on a +7.0% grade, then the near edge of the sidewalk is 0.39 feet above the elevation of

Exhibit 5-73. Increased gutter cross slope.

Exhibit 5-74. Schematic showing driveway vertical alignment concepts.

Normal curblocation

Driveway

maximum

2.0%

roadway

sidewalk

Exhibit 5-75. Example of a driveway vertical profile design.

DrivewayDriveway

maximum

8.0%maximum2.0%

roadwayMaximumbreakoversag = 9%

sidewalk Maximumbreakover

crest = 10%* Maximum breakover isthe maximum without avertical curve.

- ELSE - Vertical curve


the gutter line. The 5-ft-wide sidewalk has a +2.0% cross slope, for a rise 0.1 foot, for total riseof 0.49 feet above the gutter line elevation.

Exhibit 5-76 shows a design for a situation where the driveway would normally slope immedi-ately downward from the gutter line at a 4.33 percent grade. The alternate design (dashed line) allowsthe driveway to slope up from the gutter before sloping back down. Again, this design confines nor-mal depths of flow to the gutter, instead of allowing the gutter flow to rush down the driveway.

Other Elements

This section discusses other aspects of driveway design, such as landscaping, right-turn laneson the roadway in advance of the driveway, surface drainage in the area where the driveway meetsthe roadway and sidewalk, use of traffic control device (e.g., signs, pavement markings, and traf-fic signals), and other situations.

Landscaping and Business Signs

Appropriate landscaping near roadway-driveway-sidewalk intersections can produce envi-ronmental and aesthetic benefits. Landscaping can also directly or indirectly help meet somegeometric design objectives for one or more user groups. Landscaping can benefit drivewayusers in the following ways:

• Landscaping helps reduce stormwater run-off and soil erosion.• Tree canopies can provide shade for pedestrians.• Trees that shade pavement can reduce asphalt temperatures by as much as 36°F and fuel tank

temperatures by nearly 7°F (5-27).• Well-designed landscaping can help define driveway edges and make the driveway location

more conspicuous.

However, ill-chosen or ill-placed landscaping can be an inconvenience or even a hazard.

Tree selection and suitability should consider climate, maintenance requirements, susceptibil-ity to disease, space available for root growth, ultimate tree height, and size of mature canopies.In more extreme cases, vegetation may physically interfere with one or more driveway usergroups. Continuous maintenance of landscaping is essential to preserve plantings and sight lines,so the implications of budget limitations for maintenance should also affect landscaping decisions.Exhibit 5-77 presents suggested guidelines for the placement and control of vegetation (5-28).

Exhibit 5-76. Example design for a downhill situation.

When driveway slopes downfrom the gutter:(1) water in gutter flows intoand down the driveway;(2) vehicles pulling into theroadway must overcome thegrade.

15 ft @ 5.0%

45 ft @ 4.33%

16 ft vertical curve

30 ft @ 9.0%sidewalk in flat

part of curve May need avertical curve

roadway

gutter

With the alternate design:(1) flow is confined to gutter, upto a depth of 0.47 ft;(2) vehicles entering the streetdepart from a downhill platform.


Business signs may be present outside of the roadway right-of-way, along driveways, or withinparking areas. These signs should be placed so that they do not compete with traffic signs orobstruct sight lines of the various users.

Along a busy roadway, a business sign may help identify a driveway location. If placed close to thedriveway, a sign can help motorists who are scanning the upcoming roadside to detect the locationof the driveway they are searching for. Conversely, a business sign located far from the driveway mayactually divert a motorist’s view from the driveway location and be misleading and confusing.

Auxiliary Right-Turn Lanes

Right-turn deceleration lanes are frequently constructed to remove the slower right-turningvehicles from the through travel lanes when right-turn volumes into a driveway are heavy and/orcould have a significant adverse effect on through traffic. The benefits that accrue from havingright-turn lanes include increased capacity, reduced speed differentials and brake applications,and reduced rear-end collisions.

Exhibit 5-77. Landscaping guidelines for driveways.


Provide unobstructed lines-of-sight among bicyclists, pedestrians, and vehicles in the area

Do not install landscaping that blocks needed sight lines.

Trees should be set back a sufficient distance from the driveway – public road intersections to avoid obstructing sight lines. In urban settings, trees generally should be set back at least 20 to 30 ft on the approach to intersections and 10 to 20 ft on the far side. However, in higher speed environments, greater setbacks may be required (5-29).

Refer to the latest edition of the AASHTO Green Book for the procedure to calculate the needed distance.

The top of ground cover in driveway and street medians should not exceed 2 feet.This is 18 in. below the clear sight lineof 42 in. The bottom of the tree canopy should be at least 5 ft (60 in. high).

Landscaping should not obscure or interfere with traffic control devices or other roadside fixed objects

Vegetation should be sufficiently removed from traffic signs.

Vegetation should be sufficiently removed from utility lines.

Landscaping should not create conflicts in the paths of users

Limbs or branches that overhang any pedestrian path should be at least 7 ft above the surface of the path.

Vegetation should be sufficiently removed from pavement surfaces to prevent roots from damaging sidewalks.

Vegetation should be sufficiently removed from pavement edges to avoid scraping vehicles.

Planting arrangements should not create concealed spaces.

The ADA requires at least an 80 in. clearance above the pedestrian path (5-30).

Landscaping should not interfere with adequate artificial illumination

Trees should be set back at least 40 ft from luminaries.

Preserve an adequate roadside clear zone

Along major highways, the clear zone should normally extend at least 10 ft beyond the edge of the shoulder.

See AASHTO Roadside Design Guide (5-31).

More study is needed to better define the needed clear zone in lowerspeed, built-up urbanstreet environments.


When a pedestrian is crossing a driveway, a right-turn auxiliary lane on the public highwayallows a driver to wait without blocking a through traffic lane. A right-turn lane also removesturning vehicles from the through traffic lane, thus limiting interference with traffic progres-sion through a coordinated traffic signal system.

Right-turn lanes may be desirable, but, where provided, should not be continuous, to avoidadditional conflicts that would be introduced with both vehicular and bicycle traffic.

Installation Guidelines

The decision to provide an auxiliary right-turn deceleration lane on the roadway approachto a driveway intersection is usually made by the governing transportation agency. Although thedriveway designer may not be in a position to make a decision as to whether a right-turn decel-eration lane should be installed, it is important for the designer to have some backgroundinformation as to how a decision is made and on how provision of a right-turn decelerationlane may affect the driveway. Considerations in the decision making process generally includeroadway volumes and speeds, driveway volumes, right-turn volumes, type of traffic control atthe driveway intersection, and property availability.

Some states have established application and design criteria for right-turn deceleration lanesfor driveways and intersections on roadways under their jurisdiction, but the criteria vary widelyfrom state to state (5-32)—two examples follow.

Colorado DOT has warrants for right-turn decelerations based on roadway classificationand posted speed (5-33). For example, on a roadway classified as an Expressway, Major Bypass(Category E-X), a projected peak-hour right-turn ingress turning volume greater than 10 vphwould warrant a right-turn lane. For a Non-Rural Arterial (Category NR-A), a right-turn lanewould be warranted for any access with a projected peak-hour right-turn ingress turning vol-ume greater than 50 vph; if the posted speed is greater than 40 mph, a right-turn decelerationlane would be warranted for any access with a projected peak-hour right-turn ingress turningvolume greater than 25 vph.

Florida DOT has guidelines based on posted speed and volume (5-9, p.60). For roadwayswith a posted speed of greater than 45 mph, 35 to 55 or more right turns per hour would war-rant a right-turn lane. For roadways with a posted speed of less than or equal to 45 mph, 80 to125 or more right turns per hour would warrant a right-turn lane. The lower thresholds wouldbe most appropriate on higher volume roadways or on two-lane roadways where lateral move-ment is restricted.

The research in NCHRP Report 420 may be applied to assess the effects of right turns on curblane operations (5-12).

The installation of a right-turn deceleration lane has implications in terms of potential con-flicts with pedestrian movements. The objective of NCHRP Project 3-89, “Design Guidance forChannelized Right-Turn Lanes,” is to develop design guidance for channelized right-turn lanes,based on balancing the needs of passenger cars, trucks, buses, pedestrians (including pedestrianswith disabilities), and bicycles.

Design Considerations

An auxiliary lane for either right- or left-turn lanes should be at least 10 feet wide and, ideally,should equal that of the through lanes. If the lane has curbs, the curb face should be appropri-ately offset from the lane edge (5-1).

As shown in Exhibit 5-78, the length of the auxiliary lanes for turning vehicles consists of threecomponents: entering taper, deceleration length, and storage length (5-1). Ideally, the totallength of the auxiliary lane should be the sum of the length for these three components; however,


common practice is to accept a moderate amount of deceleration within the through lanes and toconsider the taper length as part of the deceleration length.

The following paragraphs summarize each component of the auxiliary lane length, based oninformation in the AASHTO A Policy on Geometric Design of Highways and Streets (5-1). Additionalinformation is available in this AASHTO document.

Taper. On high-speed rural roadways, a common practice has been to use a taper ratebetween 8:1 and 15:1 (longitudinal-to-transverse). In urban areas, some use a standard taperranging in length from 50 to 100 feet. The following numbers provide an example from anagency that varies the taper length according to the posted speed:

Posted speed (mph) 30 35 40 45 50 55Straight line ratios 6:1 8:1 10.5:1 12.5:1 14.5:1 16.5:1

Source: New Mexico DOT, State Access Management Manual, Ch. 8, Sec. 18, p. 92, Sep. 2001

Some considerations favor shorter tapers over longer tapers at urban intersections, includingdriveways:

1. Shorter tapers appear to produce better “targets” for approaching drivers and to give morepositive identification to an added auxiliary lane.

2. A longer taper may cause some drivers to incorrectly think that the deceleration lane is athrough lane, especially when the taper is on a horizontal curve.

3. For the same total length of taper and deceleration, a shorter taper allows the storage lengthto be longer. This results in a longer length of full-width pavement for the auxiliary lane. Theadditional storage length helps to avoid turning traffic backing up in the through travel lanesand the slower speeds during peak periods would have a shorter taper needed. However, athigher vehicle speeds, this would involve deceleration in the through or turn lane.

4. During peak periods, when the queue length in the auxiliary lane is longer, speeds may decrease,which will make a shorter taper adequate.

Deceleration Length. Provision for deceleration clear of the through traffic lanes is desirableon arterial roadways. Exhibit 5-79 lists turn lane deceleration distances from different sources.The braking distance component of stopping sight distance is included for comparison.

On many urban facilities, an auxiliary turn lane is not long enough to accommodate the stor-age and all of the deceleration within its limits. Therefore, the initial part of the deceleration takesplace in the through lanes, before the vehicle enters the auxiliary lane. In some higher volume andspeed environments, significant deceleration in the through lanes may affect safety and operationsadversely, so deceleration in the through lanes should be minimized.

For steep upgrades, a shorter deceleration length may be acceptable. For significant down-grades, the deceleration distances need to be extended.

Exhibit 5-78. Parts of a deceleration lane.


Storage Length for Left-Turn Lanes. At unsignalized driveway intersections, the storagelength, may be based on the number of turning vehicles likely to arrive in an average 2-minperiod within the peak hour. Storage for at least two passenger vehicles should be provided.Where trucks represent more than 10 percent of the traffic, storage should be sufficient for atleast one car and one truck.

Storage Length for Right-Turn Lanes. At unsignalized driveways, if the turn lane does not stopor yield to other motor vehicles, and pedestrians seldom cross the driveway, no storage may beneeded.If pedestriansoften cross the driveway, then storage for at least one vehicle may be desirable.

Drainage of Surfaces Occupied by User Groups

When there is deep standing or flowing water, the following undesirable scenarios can occur:

• A passing motor vehicle will splash nearby bicyclists, pedestrians, or persons waiting at a tran-sit stop. In the more extreme cases, it may adversely affect a driver’s ability to control a vehicle.

• Bicyclists and pedestrians are forced to wade through the water.

A good driveway design considers and accommodates the flow of water that results from sur-face runoff in a way that minimizes inconvenience to users.

Surface runoff water should flow toward a gutter, an inlet, a flume, a ditch, or other suit-able destination and not stand and pond in the roadway-driveway-sidewalk intersection area.Although it may be impossible to totally eliminate runoff, depths can be minimized and flowsdirected away from pedestrian users, the most vulnerable of the user groups.

To achieve suitable drainage and avoid creating problems, the designer should examine theamount of and direction of surface flows in and near the intersection of the driveway with theroadway and sidewalk. The designer should specify the elevations of the surfaces of the driveway,the sidewalk, and the border on the design sheets.

Drainage grates in the driveway can help intercept the surface runoff. As Exhibit 5-80 shows,if installed, grates do need to be inspected and maintained periodically to avoid creating poten-tially hazardous situations.

Exhibit 5-79. Example deceleration lengths.

Deceleration Length (ft)

Design Speed

AASHTO AASHTO Fla Fla NM NM Wis

(mph) Brakinga Value

Comfortableb Value

(Urban)c

Values(Rural)d Stope 15mphf d2g

30 86 170 -- -- 200 175 160 35 118 -- 145 -- 250 230 -- 40 154 275 155 -- 325 300 275 45 194 340 185 -- 400 370 -- 50 240 410 240 290 475 450 425 55 290 485 -- 350 550 525 --

-- indicates no value given for this speed a. braking distance (2004, p. 112) b. deceleration (2004, p. 714) c, d. assumes 10 mph speed difference (from Std. Index 301, rev. 2005). The FDOT Driveway Handbook (2005, p. 63) says “Right turn lane tapers and distances are identical to left turn lanes under stop conditions.” e, f. New Mexico State Access Mgmt. Manual, Ch. 8, Sec. 18, p. 92, Sep. 2001. One condition is deceleration to 0 mph, the other is to 15 mph. g. Wisconsin Facilities Development Manual, 11-25-1, p. 1-3, May 2006. Distance d2 is the distance traveled while the driver maneuvers laterally and stops. The values allow a 10 mph speed difference when the turning vehicle clears the through lane.


Designing to avoid directing roadway gutter flow into a driveway and onto private propertywas discussed in the Vertical Alignment section.

Traffic Controls

Signs, pavement markings, and traffic signals are called traffic control devices (TCDs). Strictlyspeaking, they are not geometric design elements, but TCDs may be used to complement a geo-metric design.

Because of low volumes and speeds, TCDs are not needed on many driveways. Driveways withmoderate to high traffic volumes are more likely to need some form of traffic control, such assigns and/or pavement markings.

Where TCDs are installed, they should be consistent with the signs and markings thatmotorists and pedestrians are familiar with, the ones they see on the surrounding roadway sys-tem. The Manual on Uniform Traffic Control Devices (MUTCD) (5-34) for streets and highwayssets forth the guidelines for the application of traffic signs, pavement markings, signals, andother TCDs.

Sign Considerations

Among the many situations that call for signs, the following are likely to be found at drivewaysand perhaps overlooked by some designers:

• Along an undivided roadway, when triangular islands (pork chops) are constructed in thedriveway entry throat to prohibit one or both left turns, installation of No Left Turn (R3-2)sign(s) in conformance with the MUTCD is needed.

• If a driveway has a wide median, drivers may find that the R4-7 Keep Right (of the mediannose) sign is helpful.

• Driveways intended for one-way operation should be accompanied by appropriate signs, somotorists will not proceed in the wrong direction. Refer to the MUTCD for information aboutthe use of One-Way, turn prohibition, and Do Not Enter signs.

If a driveway connects with a narrow roadway, motorists may find parked cars make turn-ing into or out of the driveway difficult or impossible. Some situations may call for parking

Exhibit 5-80. Damaged grates in driveway need repair.


prohibitions in advance of and past the connection and on the other side of the roadway oppo-site the connection.

Marking Considerations

The MUTCD requires that pavement markings separating opposite directions of travel, such asa center line, be yellow. Markings separating the same direction of travel (e.g., lane lines) and theouter edge lines are white, as are stop lines, crosswalk markings, and directional turn arrows.

Where driveways are wide enough to accommodate three or more lanes of traffic, pavementmarkings to delineate the intended lanes should be provided. Exhibit 5-81 shows two drive-ways wide enough for three lanes of traffic. On the driveway without the pavement markings,motorists are much more likely to position their vehicles so as to create problems and conflictswith other vehicles.

Some driveways with multiple exit lanes are marked with slightly offset stop lines, as shownin Exhibit 5-82. This is done so that when both left-turning and right-turning vehicles are try-ing to exit the driveway at the same time, the left-turning vehicle does not block the needed line-of-sight of the right-turning driver. The right-turning vehicle is given preference because a saferight-turn maneuver requires only an adequate size gap from the left, while a safe left-turn maneu-ver requires adequate size gaps from both the left and the right. This offset also accommodates thepath of a vehicle turning left from the roadway into the driveway.

Channelizing devices, such as tubular markers, have been used to enhance delineation and toreinforce turn prohibitions. They are sometimes part of a driveway triangular island installation.The MUTCD provides detailed instructions for using channelization devices, including that, ifused at night, they are to be retroreflective. Exhibit 5-83 shows tubular markers used to discour-age unwanted left turns.

Signal Considerations

Where high-volume driveways intersect public roadways, traffic signals may be necessary.Considerations are listed in Exhibit 5-84.

Driveways sometimes essentially form the fourth leg of a signalized intersection. The currentMUTCD does allow a driveway that forms the fourth approach or leg of an otherwise signalizedintersection to be unsignalized. Before deciding to exercise this option, the designer should ascer-tain that volumes and speeds on the other three approaches as well as on the driveway are lowenough so that vehicles from the unsignalized driveway can safely enter the intersection.

Exhibit 5-81. Wider driveways without and with pavement markings.

(a) (b)


Railroad Grade Crossings

Where there is a practical alternate route, it is desirable that a driveway not cross railroadtracks at grade. However, in some cases, the only access to a public road may be across a rail-road track.

The guidance in the Railroad-Highway Grade Crossing Handbook (5-35) for design of rail-highway crossings also applies to the design of driveway-rail crossings. Exhibit 5-85 lists somedesign considerations for driveways crossing railroad tracks.

Track maintenance can result in raising the track as new ballast is added to the track struc-ture. The Handbook cautions that “unless the highway profile is properly adjusted, this practicewill result in a ‘humped’ profile that may adversely affect the safety and operation of highwaytraffic over the railroad.” The greatest risk of becoming hung up at railroad-highway gradecrossings because of contact with the track or highway surface is posed by low-clearance, long-wheelbase vehicles. A similar problem can occur where the crossing is in a sag vertical curve. Inthis case, the front or rear overhangs on certain vehicles can strike or drag the pavement.

When a road parallels a railroad and an intersecting driveway crosses the railroad, a rail-road grade crossing near the roadway intersection results. The Railroad-Highway Grade

Exhibit 5-83. Example of tubular markers to prohibit a movement.

Exhibit 5-84. Traffic signal considerations.

Traffic Signal Design Element Suggested Practice Minimum green time To accommodate pedestrians crossing either the driveway or

the roadway, the designer can either establish a minimum green time that is adequate for crossing or provide pedestrian pushbuttons.

Users with disabilities Many situations, especially in urban areas, call for features such as detectable warnings to accommodate pedestrians with disabilities.

Actuation In most cases, want an actuated signal, so will not take away green from the through roadway unless there is actual driveway demand. Semi-actuated may be adequate.

Progression If the driveway traffic signal is one of a series along the roadway, then time and coordinate the signal to minimize interference with progression along the through roadway. Semi-actuated traffic signals may help minimize interruptions to through traffic on the public road.

Exhibit 5-82. Offset stop linemarkings.

yellow white

roadway

driv

eway

sidewalk

yellow white


Crossing Handbook points out that the higher occurrence of collisions at these intersectionsis due in part to a short storage area for vehicles waiting to move through the crossing andthe intersection. “If the intersection is signalized or if the driveway approach from the cross-ing is controlled by a STOP sign, queues may develop across the crossing, leading to the pos-sibility of a vehicle becoming ‘trapped’ on the crossing. Also, there are more distractions tothe motorist, leading to the possibility of vehicle-vehicle conflicts.” The critical distancebetween a driveway-rail crossing and a driveway-highway intersection is a function of thenumber and type of vehicles expected to be queued up by the intersection traffic control. Ifother viable driveway locations are available, consider routing the driveway so that it does notcross the railroad track.

References5-1. AASHTO. A Policy on Geometric Design of Highways and Streets. Washington, DC (2004) 896 pp.5-2. AASHTO. Guide for the Planning, Design, and Operation of Pedestrian Facilities. Washington, DC (July 2004)

127 pp.5-3. Louisville-Jefferson County Metro Government. Title VII: Traffic Code, Chapter 74: Bicycles and Motorcycles,

Section 74.01: Operation of Bicycles (2008) no page numbers.5-4. Guth, D., and LaDuke, R. “Veering by Blind Pedestrians: Individual Differences and Their Implications

for Instruction.” Journal of Visual Impairment and Blindness (1995) pp. 28–37.5-5. Guth, D., and Rieser, J.J. “Perception and the Control of Locomotion by Blind and Visually Impaired

Pedestrians,” in Blasch, B.B., Wiener, W.R., & Welsh, R.L. (eds.) Foundations of Orientation and Mobility(2nd ed.), AFB Press, New York (1997) pp. 317–356.

5-6. Fitzpatrick, K., et al. TCRP Report 19: Guidelines for the Location and Design of Bus Stops. TransportationResearch Board, National Research Council, Washington, DC (1996).

5-7. Gattis, J. L. “Class Notes, CVEG 4423,” Univ. of Arkansas, Fayetteville, AR (2009).5-8. Highway Capacity Manual. Transportation Research Board, National Research Council, Washington, DC

(2000).5-9. Systems Planning Office. Driveway Handbook. Florida Department of Transportation, Tallahassee, FL

(March 2005) 100 pp.5-10. Box, P.C. “Driveway Accident and Volume Studies, Part I-General Relationships.” Public Safety Systems

(May/June 1969) pp. 18–22.

Exhibit 5-85. Considerations for driveways crossing railroad tracks.

Design Element Suggested Practice Reason Intersection angle

Driveway and railroad tracks intersect at 90 degrees

Enhance the driver’s view of the crossing

Horizontalalignment - curvature

Crossings should not be located on either driveway or railroad curves

Driveway curvature limits a driver’s view of the crossing ahead and the driver’s attention may be directed toward negotiating the curve rather than looking for a train.

Railroad curvature restricts a driver’s view down the tracks from both a stopped position at the crossing and on approach to the crossing

Vertical alignment

Driveway-rail crossing should be as level as possible. AASHTO (5-1,pp. 731-733) recommends that the crossing surface be in the same plane as the top of rails for a distance of 2 feet outside the rails, and that the surface of the roadway be not more than 3 inches higher or lower than the top of the nearest rail at a point 30 feet from the rail, unless track superelevation dictates otherwise.

Improved sight distance, rideability, and braking and acceleration distances

5-11. Neuman, T.R. NCHRP Report 279: Intersection Channelization Design Guide. Transportation ResearchBoard, National Research Council, Washington, DC (1985) p. 76.

5-12. Gluck, J. S., Levinson, H. R., and Stover, V. G. NCHRP Report 420: Impacts of Access Management Techniques.Transportation Research Board, National Research Council, Washington, DC (1999) 157 pp.

5-13. Stover, V. G., and Koepke, F. J. Transportation and Land Development, 2nd edition. ITE, Washington, DC(2002) 700 pp.

5-14. Gattis, J.L., and Low, S.T. Transportation Research Record 1612, “Intersection angle geometry and thedriver’s field of view.” Transportation Research Board, National Research Council, Washington, DC(1997) pp. 10–16.

5-15. Son, Y. T., Kim, S. G., and Lee, J. K. Transportation Research Record 1796, “Methodology to calculate sightdistance available to drivers at skewed intersections.” Transportation Research Board, National ResearchCouncil, Washington, DC (2002) pp. 41–47.

5-16. Garcia, A. “Lateral vision angles and skewed intersection design.” Proceedings, Third InternationalSymposium on Highway Geometric Design (2005).

5-17. Center for Transportation Research and Education. Access Management Toolkit: Answers to frequentlyasked questions. Iowa State University, Ames, IA, http://www.ctre.iastate.edu/Research/access/toolkit/ (as ofDec. 31, 2007).

5-18. Missouri Department of Transportation. New Engineering Policy Guide, 940.16. Jefferson City, MO,http://epg.modot.org/index.php?title=940.16_Driveway_Geometrics (as of Dec. 31, 2007).

5-19. New Hampshire Department of Environmental Services. Innovative Land Use Planning Techniques: A Handbook for Sustainable Development (draft). Concord, NH, http://www.des.nh.gov/REPP/ilupth/Access_Management.doc (as of Dec. 31, 2007).

5-20. Koepke, F.J., and Levinson, H.S. NCHRP Report 348: Access Management Guidelines for Activity Centers.Transportation Research Board, National Research Council, Washington, DC (1992) pp. 94–95.

5-21. Committee on Access Management. Access Management Manual. Transportation Research Board, NationalResearch Council, Washington, DC (2003) pp. 184–185.

5-22. City of Roseville, CA. “Design Standards,” (March 2007) p. 13. http://www.roseville.ca.us/civica/filebank/blobdload.asp?BlobID=2387 (as of Oct. 28, 2008).

5-23. Kulash, W.M. Residential Streets, 3rd edition. Urban Land Institute, National Association of HomeBuilders, ASCE and ITE, Washington, DC (2001) 76 pp.

5-24. Eck, R.W., and Kang, S.K. Transportation Research Record 1327, “Low-Clearance Vehicles at Rail-HighwayGrade Crossings: An Overview of the Problem and Potential Solutions.” Transportation Research Board,National Research Council, Washington, DC (1991) pp. 27–35.

5-25. French, L.J., Clawson, A., and Eck, R.W. Transportation Research Record 1847, “Development of DesignVehicles for Hang-Up Problem.” Transportation Research Board, National Research Council, Washington,DC (2003) pp.11–19.

5-26. Eck, R.W., and Kang, S.K. Transportation Research Record 1356, “Roadway Design Standards to Accom-modate Low-Clearance Vehicles.” Transportation Research Board, National Research Council, Washington,DC (1992) pp. 80–89.

5-27. Scott, K.I., Simpson, J.R., and McPherson, E.G. “Effects of Tree Cover on Parking Lot Microclimate andVehicle Emissions.” Journal of Arboriculture, Vol. 25, No. 3 (1999) pp. 129–142.

5-28. Eck, R.W., and McGee, H. Vegetation Control for Safety: A Guide for Local Highway and Street MaintenancePersonnel. FHWA-SA-7-18, FHWA, Washington, DC (July 2007) pp.13–6, 21–23.

5-29. Schellinger, D. In Planning and Urban Design Standards. Sendich, G., Graphics Editor, American PlanningAssociation, John Wiley and Sons, New York (2006).

5-30. ADA Accessibility Guidelines for Buildings and Facilities (ADAAG), September 2002, http://www.access-board.gov/adaag/html/adaag.htm#4.4

5-31. AASHTO. Roadside Design Guide, 3rd Edition. Washington, DC (2002) 344 pp.5-32. “Lane Widths, Channelized Right Turns, and Right-Turn Deceleration Lanes in Urban and Suburban Areas.”

Contractor’s Final Report NCHRP Project 03-72, Transportation Research Board, National ResearchCouncil, Washington, DC.

5-33. Colorado Transportation Commission. Colorado State Highway Access Code. Colorado DOT, Denver,CO (1998, revised March 2002).

5-34. FHWA. Manual on Uniform Traffic Control Devices for Streets and Highways. Washington, DC (2003)760 pp.

5-35. Ogden, B.D. Railroad-Highway Grade Crossing Handbook – Revised Second Edition 2007. ITE, Washington,DC (2007) 324 pp.


Abbreviations and acronyms used without definitions in TRB publications:

AAAE American Association of Airport ExecutivesAASHO American Association of State Highway OfficialsAASHTO American Association of State Highway and Transportation OfficialsACI–NA Airports Council International–North AmericaACRP Airport Cooperative Research ProgramADA Americans with Disabilities ActAPTA American Public Transportation AssociationASCE American Society of Civil EngineersASME American Society of Mechanical EngineersASTM American Society for Testing and MaterialsATA Air Transport AssociationATA American Trucking AssociationsCTAA Community Transportation Association of AmericaCTBSSP Commercial Truck and Bus Safety Synthesis ProgramDHS Department of Homeland SecurityDOE Department of EnergyEPA Environmental Protection AgencyFAA Federal Aviation AdministrationFHWA Federal Highway AdministrationFMCSA Federal Motor Carrier Safety AdministrationFRA Federal Railroad AdministrationFTA Federal Transit AdministrationHMCRP Hazardous Materials Cooperative Research ProgramIEEE Institute of Electrical and Electronics EngineersISTEA Intermodal Surface Transportation Efficiency Act of 1991ITE Institute of Transportation EngineersNASA National Aeronautics and Space AdministrationNASAO National Association of State Aviation OfficialsNCFRP National Cooperative Freight Research ProgramNCHRP National Cooperative Highway Research ProgramNHTSA National Highway Traffic Safety AdministrationNTSB National Transportation Safety BoardPHMSA Pipeline and Hazardous Materials Safety AdministrationRITA Research and Innovative Technology AdministrationSAE Society of Automotive EngineersSAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005)TCRP Transit Cooperative Research ProgramTEA-21 Transportation Equity Act for the 21st Century (1998)TRB Transportation Research BoardTSA Transportation Security AdministrationU.S.DOT United States Department of Transportation

nchrp report 659 – guide for the geometric design of driveways

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